Amicus Curiae Brief — Suncor Energy (U.S.A.) Inc., et al., Petitioners v. County Commissioners of Boulder County, et al.

No. 25-170

In the

Supreme Court of the United States

SUNCOR ENERGY (U.S.A.) INC.; SUNCOR ENERGY SALES

INC.; EXXON MOBIL CORPORATION,

Petitioners,

v.

COUNTY COMMISSIONERS OF BOULDER COUNTY; CITY

OF BOULDER,

Respondents.

On Writ of Certiorari to the

Colorado Supreme Court

BRIEF OF AMICUS CURIAE SAVE OUR

STATES IN SUPPORT OF PETITIONERS

STEVEN A. ENGEL

MICHAEL H. MCGINLEY

DECHERT LLP

Counsel of Record

1900 K Street, NW

BRIAN A. KULP

Washington, DC 20006 ANTHONY R. JADICK

DECHERT LLP

Cira Centre

2929 Arch Street

Philadelphia, PA 19104

(215) 994-2463

michael.mcginley@dechert.com

Counsel for Amicus Curiae

May 21, 2026

i

TABLE OF CONTENTS

TABLE OF AUTHORITIES ....................................... ii

INTEREST OF AMICUS CURIAE ............................ 1

INTRODUCTION AND

SUMMARY OF ARGUMENT ................................. 2

ARGUMENT ............................................................... 4

I. The Constitution Forbids Extraterritorial

State Regulation.................................................... 4

A. The Concept of Territorial Jurisdiction

Predates the Founding .................................... 4

B. The

Constitution

Perpetuated

the

Territorial Limits of State Jurisdiction........... 7

C. This Court Has Consistently Enforced the

Territorial Limits of State Jurisdiction......... 10

II. The Decision Below Enables Colorado to

Regulate Impermissibly Beyond Its Borders. .... 15

CONCLUSION ......................................................... 20

ii

TABLE OF AUTHORITIES

Page(s)

Cases

Alden v. Maine,

527 U.S. 706 (1999) ................................................. 2

Am. Elec. Power Co. v. Connecticut,

564 U.S. 410 (2011) .......................................... 15-17

Am. Ins. Ass’n v. Garamendi,

539 U.S. 396 (2003) ............................................... 14

Baldwin v. G. A. F. Seelig, Inc.,

294 U.S. 511 (1935) ........................................... 8, 12

Banco Nacional de Cuba v. Sabbatino,

376 U.S. 398 (1964) ............................................... 14

BMW of N. Am., Inc. v. Gore,

517 U.S. 559 (1996) ..................................... 2, 11, 16

Bonaparte v. Tax Court,

104 U.S. 592 (1881) ........................................... 4, 12

Brown v. Fletcher’s Estate,

210 U.S. 82 (1908) ................................................. 12

Brown-Forman Distillers Corp. v.

N.Y. State Liquor Auth.,

476 U.S. 573 (1986) ............................................... 11

Burnet v. Brooks,

288 U.S. 378 (1933) ........................................... 4, 12

Burton’s Lessee v. Williams,

16 U.S. (3 Wheat.) 529 (1818) ................................. 8

Buvot v. Barbut,

25 Eng. Rep. 777 (Ch. 1737).................................... 6

iii

C & A Carbone, Inc. v. Town of Clarkstown,

511 U.S. 383 (1994) ............................................... 11

Camps Newfound/Owatonna, Inc. v.

Town of Harrison,

520 U.S. 564 (1997) ................................................. 9

City of Milwaukee v. Illinois,

451 U.S. 304 (1981) ................................................. 3

City of New York v. Chevron Corp.,

993 F.3d 81 (2d Cir. 2021) .......................... 15-17, 19

Coyle v. Smith,

221 U.S. 559 (1911) ............................................... 10

Denver Pub. Co. v. Bueno,

54 P.3d 893 (Colo. 2002) ........................................ 18

Edgar v. MITE Corp.,

457 U.S. 624 (1982) ............................................... 12

Franchise Tax Bd. of Cal. v. Hyatt,

587 U.S. 230 (2019) ................................. 7, 8, 12, 13

Fuld v. Palestine Liberation Org.,

606 U.S. 1 (2025) ................................... 2, 11, 16, 17

Georgia v. Pa. R.R. Co.,

324 U.S. 439 (1945) ............................................... 12

Georgia v. Tenn. Copper Co.,

206 U.S. 230 (1907) ............................................... 18

Healy v. Beer Inst., Inc.,

491 U.S. 324 (1989) ......................................... 11, 16

Hines v. Davidowitz,

312 U.S. 52 (1941) ................................................. 14

Home Ins. Co. v. Dick,

281 U.S. 397 (1930) ............................................... 12

iv

Hudson v. Guestier,

10 U.S. (6 Cranch) 281 (1810) ................................. 5

Huntington v. Attrill,

146 U.S. 657 (1892) ............................................... 11

Illinois v. City of Milwaukee,

406 U.S. 91 (1972) ................................................... 3

Int’l Paper Co. v. Ouellette,

479 U.S. 481 (1987) ............................................... 18

Kansas v. Colorado,

185 U.S. 125 (1902) ............................................... 12

Kurns v. R.R. Friction Prods. Corp.,

565 U.S. 625 (2012) ............................................... 18

McIlvaine v. Coxe’s Lessee,

8 U.S. (4 Cranch) 209 (1808) ................................... 4

Murphy v. NCAA,

584 U.S. 453 (2018) ................................................. 7

N.Y. Life Ins. Co. v. Head,

234 U.S. 149 (1914) ............................................... 11

Nat’l Pork Producers Council v. Ross,

598 U.S. 356 (2023) ........................................... 3, 17

New State Ice Co. v. Liebmann,

285 U.S. 262 (1932) ............................................... 17

Nielsen v. Oregon,

212 U.S. 315 (1909) ............................................... 12

Osborn v. Ozlin,

310 U.S. 53 (1940) ................................................. 19

Pennoyer v. Neff,

95 U.S. (5 Otto) 714 (1877).................................... 11

v

Pollard v. Hagan,

44 U.S. (3 How.) 212 (1845) ................................... 10

PPL Mont., LLC v. Montana,

565 U.S. 576 (2012) ................................................. 2

Rhode Island v. Massachusetts,

37 U.S. (12 Pet.) 657 (1838) ............................ 10, 13

Rose v. Himely,

8 U.S. (4 Cranch) 241 (1807) ................................... 5

San Diego Bldg. Trades Council v. Garmon,

359 U.S. 236 (1959) ............................................... 18

Seila Law LLC v. CFPB,

591 U.S. 197 (2020) ................................................. 4

State Farm Mut. Auto Ins. Co. v. Campbell,

538 U.S. 408 (2003) ................................................. 3

Tex. Indus., Inc. v. Radcliff Materials, Inc.,

451 U.S. 630 (1981) ......................................... 13, 18

The Antelope,

23 U.S. (10 Wheat.) 66 (1825) ................................. 5

The Schooner Exch. v. McFaddon,

11 U.S. (7 Cranch) 116 (1812) .............................. 4-7

Triquet v. Bath,

3 Burr. 1478 (K.B. 1764) ......................................... 6

United States v. Belmont,

301 U.S. 324 (1937) ............................................... 14

United States v. Bevans,

16 U.S. (3 Wheat.) 336 (1818) ............................... 10

United States v. Pink,

315 U.S. 203 (1942) ............................................... 14

vi

Ware v. Hylton,

3 U.S. (3 Dall.) 199 (1796) ....................................... 7

Watson v. Emps. Liab. Assurance Corp.,

348 U.S. 66 (1954) ................................................. 12

World-Wide Volkswagen Corp. v. Woodson,

444 U.S. 286 (1980) ............................................... 12

Zschernig v. Miller,

389 U.S. 429 (1968) ............................................... 14

Constitutional Provisions

U.S. Const. art. I, § 8, cl. 3 ......................................... 9

U.S. Const. art. I, § 10 .......................................... 7, 12

U.S. Const. art. IV, § 2, cl. 2 ....................................... 8

U.S. Const. art. IV, § 3, cl. 1 ....................................... 9

U.S. Const. art. VI, cl. 2 ............................................. 8

U.S. Const. amend. XIV, § 1 ....................................... 8

Other Authorities

4 William Blackstone, Commentaries on the Law

of England (1769) .................................................... 6

Thomas M. Cooley, A Treatise on the

Constitutional Limitations Which Rest upon

the Legislative Power of the States of the

American Union (1868) ................................... 10, 11

Declaration of Independence (U.S. 1776) .................. 4

Environmental Protection Agency,

Endangerment and Cause or Contribute

Findings for Greenhouse Gases Under Section

202(a) of the Clean Air Act, 74 Fed. Reg.

66,496 (Dec. 15, 2009) ........................................... 15

vii

Katherine Florey, State Courts, State Territory,

State Power: Reflections on the

Extraterritoriality Principle in Choice of Law

& Legislation, 84 Notre Dame L. Rev. 1057

(2009) ....................................................................... 9

Jack L. Goldsmith & Curtis A. Bradley, The

Current Illegitimacy of International Human

Rights Litigation, 66 Fordham L. Rev. 319

(1997) ....................................................................... 6

1 James Kent, Commentaries on American Law

(William M. Lacy ed., 1889) .................................... 7

Douglas Laycock, Equal Citizens of Equal and

Territorial States: The Constitutional

Foundations of Choice of Law, 92 Colum. L.

Rev. 249 (1992) ........................................................ 9

Letter from Thomas Jefferson to Mr. Wythe

(Sept. 16, 1787), in 2 Memoirs,

Correspondence, and Private Papers of

Thomas Jefferson 228 (Thomas Jefferson

Randolph ed., 1829) ............................................... 13

Gillian E. Metzger, Congress, Article IV, and

Interstate Relations, 120 Harv. L. Rev. 1468

(2007) ....................................................................... 8

William Rawle, A View of the Constitution of the

United States of America (1825) ............................. 6

Donald H. Regan, Siamese Essays: (I) CTS Corp.

v. Dynamics Corp. of America and Dormant

Commerce Clause Doctrine; (II)

Extraterritorial State Legislation, 85 Mich. L.

Rev. 1865 (1987) ...................................................... 9

viii

Joseph Story, Commentaries on the Conflict of

Laws (1834) ............................................................. 5

Zephyr Teachout, Defining and Punishing

Abroad: Constitutional Limits on the

Extraterritorial Reach of the Offenses Clause,

48 Duke L.J. 1305 (1998) ........................................ 6

The Articles of Confederation of the United

Colonies of New England (May 19, 1643),

The Avalon Project, bit.ly/3OMfKJh ....................... 7

The Federalist No. 6 (Alexander Hamilton)

(Clinton Rossiter ed., 2003)..................................... 8

The Federalist No. 39 (James Madison)

(Clinton Rossiter ed., 2003)..................................... 2

The Federalist No. 42 (James Madison)

(Clinton Rossiter ed., 2003)............................... 9, 13

The Federalist No. 45 (James Madison)

(Clinton Rossiter ed., 2003)..................................... 2

The Federalist No. 80 (Alexander Hamilton)

(Clinton Rossiter ed., 2003)............................. 13, 14

Emer de Vattel, The Law of Nations

(Joseph Chitty ed., 1883) ..................................... 4-6

1

INTEREST OF AMICUS CURIAE1

Save Our States Action (“Save Our States”) is a

project of People for Opportunity, a 501(c)(4) nonprofit. The project is devoted to protecting freedom for

Americans through federalism. To that end, Save Our

States has focused its advocacy and educational

initiatives on issues where the balance between local

control and national powers is essential. From

defending the Electoral College to protecting our

economy, elections, and health care from government

overreach, Save Our States works to ensure that

America’s system serves citizens, not special interests

or distant bureaucracies.

Save Our States has a significant interest in this

case. Our Constitution creates a system of horizontal

federalism in which the States are coequal sovereigns.

From that bedrock principle, it follows that each

State’s sovereignty is confined to its borders. The

Framers thus understood that no State can regulate

beyond its territorial limits. But the decision below

authorizes just that. It allows a single State—indeed,

a single Colorado municipality—to “prosecute statelaw claims that will both effectively regulate

interstate air pollution and have more than an

incidental effect on foreign affairs.” Pet.App.25a

(Samour, J., dissenting). Save Our States submits this

brief to explain why the Constitution forbids that state

exercise of extraterritorial authority.

No counsel for any party authored this brief in whole or in part

and no entity or person, aside from amicus curiae, its members,

or its counsel, made any monetary contribution intended to fund

the preparation or submission of this brief.

1

2

INTRODUCTION AND

SUMMARY OF ARGUMENT

Federalism is a defining feature of our Republic.

This “system established by our Constitution

preserves the sovereign status of the States.” Alden v.

Maine, 527 U.S. 706, 714 (1999). And, pursuant to

that design, each State retains “numerous and

indefinite” powers to promote “the internal order,

improvement, and prosperity of the State.” The

Federalist No. 45, at 289 (James Madison) (Clinton

Rossiter ed., 2003).

At the same time, these broad state police powers

are subject to strict geographic limits. The States can

legislate only “within their respective spheres.” The

Federalist No. 39, at 241 (James Madison). And “[t]he

Constitution confers upon the Federal Government—

and it alone—both nationwide and extraterritorial

authority.” Fuld v. Palestine Liberation Org., 606 U.S.

1, 15 (2025). In that way, “[s]tate sovereign authority

is bounded by the States’ respective borders.” Id. at

14. Indeed, that concept of territorial jurisdiction

predates and is embedded in our constitutional design.

As a result, no State can deploy its lawmaking

power within its territorial boundaries to dictate

policy on global issues for the fifty “coequal

sovereigns” that make up the United States. PPL

Mont., LLC v. Montana, 565 U.S. 576, 591 (2012). Nor

can any State regulate beyond its jurisdiction to

“impose its own policy choice on neighboring States”—

whether through prospective legislation or retroactive

damages awards. BMW of N. Am., Inc. v. Gore, 517

U.S. 559, 571–72 & n.17 (1996). Instead, “each State

may make its own reasoned judgment about what

3

conduct is permitted or proscribed within its borders.”

State Farm Mut. Auto Ins. Co. v. Campbell, 538 U.S.

408, 422 (2003).

Those foundational principles should resolve this

case. The City of Boulder has brought this suit as a

means to regulate the emission of greenhouse gases all

over the world. Using Colorado state law, the City

seeks to punish Petitioners for the role that their

fossil-fuel activities allegedly played in climate

change. But, as all agree, climate change presents an

issue of “global import.” Pet.App.1a. As such, “state

law cannot be used” to impose liability for the out-ofstate conduct alleged here. City of Milwaukee v.

Illinois, 451 U.S. 304, 313 n.7 (1981) (“Milwaukee II”).

Rather, federal law is the “necessary” and exclusive

source for asserting “the environmental rights of a

State against improper impairment by sources outside

its domain.” Illinois v. City of Milwaukee, 406 U.S. 91,

107 n.9 (1972) (“Milwaukee I”) (citation omitted).

The lower court’s contrary decision enables

Colorado to regulate beyond its borders. And that

transgresses the “territorial limits of state authority

under the Constitution’s horizontal separation of

powers.” Nat’l Pork Producers Council v. Ross, 598

U.S. 356, 376 n.1 (2023). The Constitution thus

preempts this suit, regardless of whether the Clean

Air Act does the same.

This Court should reverse.

4

ARGUMENT

I. The Constitution Forbids Extraterritorial

State Regulation.

This Court has long held that “[n]o State can

legislate except with reference to its own jurisdiction.”

Bonaparte v. Tax Court, 104 U.S. 592, 594 (1881).

That geographic limit on sovereignty—known as

“territorial jurisdiction”—traces back to pre-Founding

understandings of the law of nations. The Schooner

Exch. v. McFaddon, 11 U.S. (7 Cranch) 116, 137 (1812)

(Marshall, C.J.). And that concept took on heightened

importance when the Framers “‘split the atom of

sovereignty’ itself into one Federal Government and

the States.” Seila Law LLC v. CFPB, 591 U.S. 197,

223 (2020) (citation omitted). “The bond of the

Constitution qualifies th[e] jurisdiction” of the States

even further than the law of nations otherwise would.

Burnet v. Brooks, 288 U.S. 378, 401 (1933).

A. The Concept of Territorial Jurisdiction

Predates the Founding.

1. The territorial limits of the States’ authority

derive from the eighteenth-century law of nations.

After all, “[t]he law of nations is the law of sovereigns.”

Emer de Vattel, The Law of Nations, bk. 1, ch. 1, § 12

(Joseph Chitty ed., 1883). And the States “became

entitled, from the time when they declared themselves

independent, to all the rights and powers of sovereign

states.” McIlvaine v. Coxe’s Lessee, 8 U.S. (4 Cranch)

209, 212 (1808). That is, they could “do all . . . Acts

and Things which Independent States may of right do”

consistent with the law of nations. Declaration of

Independence ¶ 32 (U.S. 1776).

5

By the Founding, the law of nations had developed

to recognize the equality of sovereign states. As Emer

de Vattel explained, “[e]very nation that governs itself,

under what form soever, without dependence on any

foreign power, is a Sovereign State.” Vattel, supra, at

bk. 1, ch. 1, § 4 (italics omitted). And “[i]ts rights are

naturally the same as those of any other state.” Id.

That means that no sovereign state could “naturally

lay claim to any superior prerogative,” and “[p]ower or

weakness [did] not in this respect produce any

difference.” Id. at intro., § 18, bk. 2, ch. 3, § 36.

Indeed, “[n]o principle of general law [was] more

universally acknowledged” during the Founding-era

than this “perfect equality of nations.” The Antelope,

23 U.S. (10 Wheat.) 66, 122 (1825) (Marshall, C.J.).

From this basic principle of coequal sovereignty, it

followed that no nation could “rightfully impose a rule

on another.” Id. “Each legislates for itself, but its

legislation can operate on itself alone.” Id. As a result,

“all the laws made by a sovereign have no force or

authority except within the limits of his domains.”

Joseph Story, Commentaries on the Conflict of Laws 8

(1834). And “no state or nation can, by its laws,

directly affect, or bind property out of its own territory,

or persons not resident therein.” Id. at 21. “The

jurisdiction of the nation within its own territory is

necessarily exclusive and absolute.”

Schooner

Exchange, 11 U.S. (7 Cranch) at 136; see also Rose v.

Himely, 8 U.S. (4 Cranch) 241, 279 (1807) (“[T]he

legislation of every country is territorial; that beyond

its own territory, it can only affect its own subjects or

citizens.”), overruled on other grounds by Hudson v.

Guestier, 10 U.S. (6 Cranch) 281, 285 (1810).

6

This territorial understanding of sovereignty

pervaded international law by the Founding era. So

much so that “extraterritorial regulation would have

been unthinkable in the eighteenth century.” Jack L.

Goldsmith & Curtis A. Bradley, The Current

Illegitimacy of International Human Rights

Litigation, 66 Fordham L. Rev. 319, 361 (1997).

Jurisdiction “almost never extended beyond a nation’s

borders.” Zephyr Teachout, Defining and Punishing

Abroad: Constitutional Limits on the Extraterritorial

Reach of the Offenses Clause, 48 Duke L.J. 1305, 1317

(1998). The “few exceptions” were for cases involving

ambassadors, “[m]aritime jurisdiction, consent, and

some cases of treachery abroad” by a nation’s citizens.

Id. Absent these “peculiar circumstances”—in which

the theoretical basis for territorial jurisdiction broke

down—the sovereigns of the world had “absolute and

complete jurisdiction within their respective

territories.” Schooner Exchange, 11 U.S. (7 Cranch) at

136; see also Vattel, supra, at bk. 2, ch. 7, § 84.

2. The law of nations heavily influenced the

Framers. It was widely regarded as “part of the law of

England.” Triquet v. Bath, 3 Burr. 1478, 1481 (K.B.

1764) (quoting Buvot v. Barbut, 25 Eng. Rep. 777, 778

(Ch. 1737)). And it was “adopted in its full extent by

the common law” as “the law of the land.” 4 William

Blackstone, Commentaries on the Law of England 67

(1769). This acceptance of the law of nations enabled

England to “be a part of the civilized world.” Id.

That embrace carried over to the United States.

See, e.g., William Rawle, A View of the Constitution of

the United States of America 104 (1825). In fact, it

took root in the colonies long before independence.

7

The New England colonies, for instance, specifically

endorsed the principle of territorial jurisdiction in

1643, agreeing that each “shall have . . . peculiar

jurisdiction and government within their limits.” The

Articles of Confederation of the United Colonies of

New England § 3 (May 19, 1643), The Avalon Project,

bit.ly/3OMfKJh. Then, “[w]hen the United States

declared their independence, they were bound to

receive the law of nations, in its modern state of purity

and refinement.” Ware v. Hylton, 3 U.S. (3 Dall.) 199,

281 (1796) (Wilson, J.); accord 1 James Kent,

Commentaries on American Law 1–2 (William M. Lacy

ed., 1889). That included the doctrine of territorial

jurisdiction. See Schooner Exchange, 11 U.S. (7

Cranch) at 136.

B. The

Constitution

Perpetuated

the

Territorial Limits of State Jurisdiction.

1. The Constitution embraced this geographic

understanding of sovereign authority in the relations

of each of the United States to the other. Indeed, “[t]he

Constitution limits state sovereignty” even further

than the law of nations “in several ways.” Murphy v.

NCAA, 584 U.S. 453, 470 (2018) (emphasis added).

Article I, Section 10 “directly prohibits the States from

exercising some attributes of sovereignty,” including

on the international stage. Id. (citing U.S. Const. art.

I, § 10). Article IV “imposes duties on the States not

required by international law.” Franchise Tax Bd. of

Cal. v. Hyatt, 587 U.S. 230, 245 (2019). Certain

“grants of power to the Federal Government” create

“implicit restrictions on the States.” Murphy, 584 U.S.

at 470. And the Supremacy Clause subordinates state

8

law to the duly enacted “laws of the United States.”

U.S. Const. art. VI, cl. 2.

Those limits on state sovereignty were necessary to

transform “the States from a loose league of friendship

into a perpetual Union.” Hyatt, 587 U.S. at 246. The

Framers had learned from experience of the States’

inclination “to aggrandize themselves at the expense

of their neighbors.” The Federalist No. 6, at 54

(Alexander Hamilton) (citation omitted). And so,

“happily for our domestic harmony, the power of

aggressive operation against each other [was] taken

away.” Burton’s Lessee v. Williams, 16 U.S. (3 Wheat.)

529, 538 (1818). Upon ratification, the States “no

longer relate[d] to each other solely as foreign

sovereigns.” Hyatt, 587 U.S. at 245. Instead, they

framed a Republic “upon the theory that the peoples

of the several states must sink or swim together.”

Baldwin v. G. A. F. Seelig, Inc., 294 U.S. 511, 523

(1935).

2. Consistent with that design, nothing in the

Constitution expands the States’ powers to regulate

beyond their borders. If anything, “[t]he principle that

states are territorially bound . . . permeates the

Constitution.” Gillian E. Metzger, Congress, Article

IV, and Interstate Relations, 120 Harv. L. Rev. 1468,

1520 (2007). The Extradition Clause, for example,

presupposes territorial limits on State authority when

it speaks of the “State having Jurisdiction of the

Crime.” U.S. Const. art. IV § 2, cl. 2. The Fourteenth

Amendment prohibits the States from “deny[ing] to

any person within its jurisdiction the equal protection

of the laws.” U.S. Const. amend. XIV, § 1 (emphasis

added). The New States Clause protects the States

9

from having to cede territory “within the[ir]

Jurisdiction.” U.S. Const. art. IV, § 3, cl. 1. And the

Commerce Clause “restrain[s]” the “authority of the

States” to regulate commerce that crosses their

borders. The Federalist No. 42, at 263 (James

Madison); see U.S. Const. art. I, § 8, cl. 3. So does the

Import-Export Clause—at least as that Clause was

originally

understood.

See

Camps

Newfound/Owatonna, Inc. v. Town of Harrison, 520

U.S. 564, 621–37 (1997) (Thomas, J., dissenting).

These specific provisions reflect more “general

structural principles of horizontal federalism.”

Katherine Florey, State Courts, State Territory, State

Power: Reflections on the Extraterritoriality Principle

in Choice of Law & Legislation, 84 Notre Dame L. Rev.

1057, 1060 (2009). That is, the “extraterritoriality

principle” is “one of those foundational principles of

our federalism” that we can “infer from the structure

of the Constitution as a whole.” Donald H. Regan,

Siamese Essays: (I) CTS Corp. v. Dynamics Corp. of

America and Dormant Commerce Clause Doctrine; (II)

Extraterritorial State Legislation, 85 Mich. L. Rev.

1865, 1885 (1987).

Other sources of law reinforce that understanding.

Most States have “territorial definitions” that “are

specified in state organic acts” and “enabling acts.”

Douglas Laycock, Equal Citizens of Equal and

Territorial States: The Constitutional Foundations of

Choice of Law, 92 Colum. L. Rev. 249, 317 (1992). And

“for the older states,” these boundaries are expressed

“in treaties, territorial cessions, and royal grants.” Id.

at 317–18. Thus, “territory is part of the very

definition of the state[s].” Id. at 316. Their territorial

10

boundaries define the limits of their sovereignty. And

that is the way the States have governed alongside one

another since the dawn of our Republic.

C. This Court Has Consistently Enforced the

Territorial Limits of State Jurisdiction.

1. This Court first recognized the territorial limits

of state authority nearly two centuries ago. In Rhode

Island v. Massachusetts, the Court explained that in

our constitutional system, no State can “have any

right beyond its territorial boundary.” 37 U.S. (12

Pet.) 657, 733 (1838). The Court thus held, “without

hesitation, [that] the jurisdiction of a state is

coextensive with its territory.” Id. (quoting United

States v. Bevans, 16 U.S. (3 Wheat.) 336, 386–87

(1818)).

Just a few years later, the Court reiterated that

core tenet of horizontal federalism. It emphasized

that every State is “entitled to the sovereignty and

jurisdiction over all the territory within her limits.”

Pollard v. Hagan, 44 U.S. (3 How.) 212, 228 (1845).

But that sovereignty runs out at the border—at which

point another State’s sovereignty takes over. To hold

otherwise would deny neighboring States their “equal

footing” in the Union. Id. at 229. And that would

jeopardize “the harmonious operation of the scheme

upon which the Republic was organized.” Coyle v.

Smith, 221 U.S. 559, 580 (1911).

These territorial limits of state sovereignty became

firmly entrenched by the Civil War. As one influential

treatise explained, “[t]he legislative authority of every

State must spend its force within the territorial limits

of the State.” Thomas M. Cooley, A Treatise on the

Constitutional Limitations Which Rest upon the

11

Legislative Power of the States of the American Union

127–28 (1868). No State can “make laws by which

people outside the State must govern their actions.”

Id. at 128.

2. Nothing has changed since. Shortly after the

Civil War, this Court reaffirmed that “[t]he several

States are of equal dignity and authority, and the

independence of one implies the exclusion of power

from all others.” Pennoyer v. Neff, 95 U.S. (5 Otto) 714,

722 (1877). As such, “the laws of one State have no

operation outside of its territory.” Id. In Huntington

v. Attrill, this Court again held that “[l]aws have no

force of themselves beyond the jurisdiction of the State

which enacts them, and can have extra-territorial

effect only by the comity of other States.” 146 U.S.

657, 669 (1892). So too in New York Life Insurance Co.

v. Head, where the Court explained that it “would be

impossible to permit the statutes of [one State] to

operate beyond the jurisdiction of that State.” 234

U.S. 149, 161 (1914).

To allow such an extraterritorial assertion of state

power would “throw[] down the constitutional barriers

by which all the States are restricted within the orbits

of their lawful authority and upon the preservation of

which the Government under the Constitution

depends.” Id. “This is so obviously the necessary

result of the Constitution that it has rarely been called

in question.” Id. And an unbroken line of this Court’s

decisions confirms this understanding. See, e.g., Fuld,

606 U.S. at 14; Gore, 517 U.S. at 571; C & A Carbone,

Inc. v. Town of Clarkstown, 511 U.S. 383, 393 (1994);

Healy v. Beer Inst., Inc., 491 U.S. 324, 336 (1989);

Brown-Forman Distillers Corp. v. N.Y. State Liquor

12

Auth., 476 U.S. 573, 583–85 (1986); Edgar v. MITE

Corp., 457 U.S. 624, 642–43 (1982) (plurality op.);

World-Wide Volkswagen Corp. v. Woodson, 444 U.S.

286, 293 (1980); Watson v. Emps. Liab. Assurance

Corp., 348 U.S. 66, 70 (1954); Baldwin, 294 U.S. at

521; Burnet, 288 U.S. at 404; Home Ins. Co. v. Dick,

281 U.S. 397, 410 (1930); Nielsen v. Oregon, 212 U.S.

315, 321 (1909); Brown v. Fletcher’s Estate, 210 U.S.

82, 89 (1908); Bonaparte, 104 U.S. at 594.

The upshot of these decisions is simple: In our

federalist system, each State may regulate only “with

reference to its own jurisdiction.” Bonaparte, 104 U.S.

at 594. And no State has the “power to project” its law

into another to control or punish conduct beyond its

borders. Baldwin, 294 U.S. at 521. Such an exercise

of extraterritorial power would violate the “equal

dignity and sovereignty” of other States that is central

to our constitutional scheme. Hyatt, 587 U.S. at 245.

3. A corollary of the States’ territorial jurisdiction

is that federal law must govern disputes concerning

interstate matters.

After all, “[t]he traditional

methods available to a sovereign for the settlement” of

such cross-border disputes “were diplomacy and war.”

Georgia v. Pa. R.R. Co., 324 U.S. 439, 450 (1945). But

the Constitution explicitly stripped the States of those

sovereign tools. See U.S. Const. art. I, § 10. The

States cannot “make war upon each other,” nor can

they “enter upon diplomatic relations” through

interstate compacts absent the consent of Congress.

Kansas v. Colorado, 185 U.S. 125, 143 (1902). And

States cannot exercise “raw power to apply their own

law to such matters” either. Hyatt, 587 U.S. at 246.

13

Rather, the authority to govern interstate matters

is reserved to the federal government. That is an

indispensable element of horizontal federalism:

“[O]ur federal system does not permit [a] controversy

to be resolved under state law” where “the

interstate . . . nature of the controversy makes it

inappropriate for state law to control.” Tex. Indus.,

Inc. v. Radcliff Materials, Inc., 451 U.S. 630, 641

(1981). Instead, “[w]hatever practices may have a

tendency to disturb the harmony between the States

are proper objects of federal superintendence and

control.” The Federalist No. 80, at 476 (Alexander

Hamilton). Or, as Thomas Jefferson put it in 1787, in

describing the proposed Constitution: The States may

“severally preserve their sovereignty in whatever

concerns themselves alone,” but “whatever may

concern another State, or any foreign nation, should

be made a part of the federal sovereignty.” Letter from

Thomas Jefferson to Mr. Wythe (Sept. 16, 1787), in 2

Memoirs, Correspondence, and Private Papers of

Thomas Jefferson 228, 230 (Thomas Jefferson

Randolph ed., 1829).

Accordingly, these cross-jurisdictional “subjects

that were decided by pure ‘political power’ before

ratification now turn on federal ‘rules of law.’” Hyatt,

587 U.S. at 246 (quoting Rhode Island, 37 U.S. (12

Pet.) at 737).

4. This rule has particular force for subjects with

global implications. “If we are to be one nation in any

respect, it clearly ought to be in respect to other

nations.” The Federalist No. 42, at 260 (James

Madison). To that end, the Framers devised a system

in which sovereign “[p]ower over external affairs is not

14

shared by the States.” United States v. Pink, 315 U.S.

203, 233 (1942). That authority is instead “vested in

the national government exclusively.” Id.; see also

United States v. Belmont, 301 U.S. 324, 330 (1937).

And it must “be left entirely free from local

interference.” Hines v. Davidowitz, 312 U.S. 52, 63

(1941).

Given that division of authority, state “regulations

must give way if they impair the effective exercise of

the Nation’s foreign policy.” Zschernig v. Miller, 389

U.S. 429, 440 (1968). And the federal government

need not have expressed a formal policy for this

preemption to apply. The “absence of a treaty” or

federal statute will not salvage a state law that “may

disturb foreign relations” nevertheless. Id. at 441; see

also Banco Nacional de Cuba v. Sabbatino, 376 U.S.

398, 424–27 (1964). That is because preemption flows

from the very structure of the Constitution itself—

from its “allocation of the foreign relations power to

the National Government.”

Am. Ins. Ass’n v.

Garamendi, 539 U.S. 396, 413 (2003).

This structural limit on state authority ensures

that when “embracing our relations with foreign

nations, we are but one people, one nation, one power.”

Hines, 312 U.S. at 63 (citation omitted). “The Union

will undoubtedly be answerable to foreign powers for

the conduct of its members.” The Federalist No. 80, at

475 (Alexander Hamilton). And the “peace of the

whole ought not to be left at the disposal of a part.” Id.

(capitalization altered). Only a uniform federal rule of

decision can apply.

15

II. The Decision Below Enables Colorado to

Regulate Impermissibly Beyond Its Borders.

A. The decision below defies these basic principles

of our constitutional order. The City of Boulder seeks

to hold Petitioners liable for “the role that [their]

production, promotion, refining, market, and sale of

fossil fuels has allegedly played in exacerbating

climate change.” Pet.App.2a. The Colorado Supreme

Court allowed those claims to “proceed under state

law.” Pet.App.1a. But those claims are beyond

Colorado’s constitutional reach. They are, after all,

“based on harms the State of Colorado has allegedly

suffered as a result of global climate change.”

Pet.App.25a (Samour, J., dissenting). Nothing about

that global phenomenon is specific to the Centennial

State. In fact, the claims here seek “damages for the

cumulative

impact

of

conduct

occurring

simultaneously across just about every jurisdiction on

the planet.” City of New York v. Chevron Corp., 993

F.3d 81, 92 (2d Cir. 2021).

That is the very nature of a challenge, like this one,

premised on the environmental effects of greenhouse

gas emissions. “Greenhouse gases once emitted

‘become well mixed in the atmosphere.’” Am. Elec.

Power Co. v. Connecticut, 564 U.S. 410, 422 (2011)

(quoting

Environmental

Protection

Agency,

Endangerment and Cause or Contribute Findings for

Greenhouse Gases Under Section 202(a) of the Clean

Air Act, 74 Fed. Reg. 66,496, 66,514 (Dec. 15, 2009)).

And Colorado is not sealed off from the world by a

bubble. Carbon emissions from China, Cambodia, or

Canada affect climate change in Colorado (and

16

elsewhere) just like emissions from California,

Colorado, or Connecticut. See id.

The “practical effect” of this lawsuit is therefore “to

control conduct beyond the boundaries of the state.”

Healy, 491 U.S. at 336. Indeed, the only way to avoid

the City’s theory of liability is to reduce—or even

completely halt—the production and sale of fossil fuels

“across every state (and country).” City of New York,

993 F.3d at 92. No matter whether such activities are

lawful in other cities beyond Boulder. No matter

whether such activities are lawful in other States

beyond Colorado. And no matter whether such

activities are lawful in other countries beyond the

United States. Colorado seeks to regulate those

activities across the entire globe through state tort

law.

That assertion of raw power far exceeds the limits

of Colorado’s territorial jurisdiction. Colorado “may

not impose economic sanctions” through its own laws

to “chang[e] [Petitioners’] lawful conduct in other

States.” Gore, 517 U.S. at 572. And it certainly cannot

do so to deter and punish lawful activities performed

in other countries. Only the federal government may

exert this “nationwide and extraterritorial authority.”

Fuld, 606 U.S. at 15.

To be clear, this constitutional limitation does not

leave Colorado powerless to regulate Petitioners’

statewide activities or to reduce greenhouse gas

emissions within Colorado’s territory—to the extent

not preempted by federal legislation. “It is one of the

happy incidents of the federal system that a single

courageous State may, if its citizens choose, serve as a

laboratory; and try novel social and economic

17

experiments” to promote the transition away from

fossil fuels. New State Ice Co. v. Liebmann, 285 U.S.

262, 311 (1932) (Brandeis, J., dissenting). Those statelevel experiments could eventually prompt (or

dissuade) federal legislation to curtail the use of fossil

fuels. But if Colorado wants to conduct such an

experiment, then it must do so “without risk to the rest

of the country” or our international relations—by

limiting the scope of its rule and punishment to the

State’s own jurisdiction. Id. It cannot impose its view

of environmental policy on the world by exceeding the

“territorial limits of state authority.” Pork Producers,

598 U.S. at 376 n.1. The lower court erred in holding

to the contrary.

B. The lower court’s rejoinders lack merit. The

court started off by noting that the passage of the

Clean Air Act “displaced the federal common law of

nuisance.” Pet.App.10a. But such displacement is

irrelevant to the constitutional division of authority

between the federal and state governments. Colorado

“state law does not suddenly become . . . competent to

address issues that demand a unified federal standard

simply because Congress saw fit to displace a federal

court-made standard with a legislative one.” City of

New York, 993 F.3d at 98. Federal statute or not,

Colorado’s sovereign authority remains “bounded by

[its] borders.” Fuld, 606 U.S. at 14. The decision

below elides that key point.

Nor does anything else in the lower court’s decision

make up for that mistake. The majority appeared to

admit that federal law must govern “suits brought by

one State to abate pollution emanating from another

State.” Pet.App.17a (quoting Am. Elec. Power, 564

18

U.S. at 421). The States “by their union made the

forcible abatement of outside nuisances impossible to

each.” Georgia v. Tenn. Copper Co., 206 U.S. 230, 237

(1907). But, the majority emphasized, the City of

Boulder wants a different remedy here. Its suit “seeks

damages” for “harms stemming from the production

and sale of fossil fuels.” Pet.App.17a (emphasis

added).

That is an empty distinction.

After all,

“[r]egulation can be as effectively exerted through an

award of damages as through some form of preventive

relief.” San Diego Bldg. Trades Council v. Garmon,

359 U.S. 236, 247 (1959). In tort law, an “obligation to

pay compensation” is indeed “designed to be” a “potent

method of governing conduct and controlling policy.”

Kurns v. R.R. Friction Prods. Corp., 565 U.S. 625, 637

(2012) (emphasis added; citation omitted).

The

monetary liability “deters the socially wrongful

conduct in the first place.” Denver Pub. Co. v. Bueno,

54 P.3d 893, 898 (Colo. 2002). And it serves to alter

the defendant’s behavior going forward. See id.

This case proves the point. If damages are

awarded here, then Petitioners will have to “change

[their] methods of doing business and controlling

pollution to avoid the threat of ongoing liability.” Int’l

Paper Co. v. Ouellette, 479 U.S. 481, 495 (1987). The

City’s damages remedy thus cannot mask the

regulatory aims of this lawsuit. Nor can it change the

fact that the “interstate or international nature of

[this] controversy makes it inappropriate for state law

to control.” Tex. Indus., 451 U.S. at 641.

Finally, the lower court believed that the City’s

“claims do not seek to regulate [greenhouse gas]

19

emissions” at all, because the complaint “focus[es] on

defendants’ upstream production” of fossil fuels.

Pet.App.21a. But the City’s “[a]rtful pleading” in this

regard “cannot transform [its] complaint into

anything other than a suit over global greenhouse gas

emissions.” City of New York, 993 F.3d at 91. “It is

precisely because fossil fuels emit greenhouse gases”

around the world—which are alleged to have

collectively brought about climate change—that “the

City is seeking damages.” Id.; see also Pet.App.33a

(Samour, J., dissenting) (“[T]he bottom line is that this

suit is about the alleged [greenhouse gas] emissions

from the energy companies, even if the energy

companies are actually a few steps removed from the

physical release of the pollutants.”).

In any event, the City’s feigned focus on upstream

activities makes no difference in the constitutional

calculus. If anything, attacking Petitioners’ upstream

production

activities

only

underscores

the

extraterritoriality problem.

As “multinational”

companies, Petitioners’ upstream production activities

span the globe. See, e.g., JA14 (“Suncor Energy is the

parent company of a multinational, integrated oil and

gas enterprise[.]”); JA21 (“Exxon is a multinational,

vertically integrated, fossil fuel company.”).

Those multinational upstream activities fall well

outside Colorado’s territorial jurisdiction—just like

the global greenhouse gas emissions that this lawsuit

is really about. Either way, Colorado has “reached

beyond her borders to regulate a subject which was

none of her concern because the Constitution has

placed control elsewhere.” Osborn v. Ozlin, 310 U.S.

20

53, 62 (1940). That it cannot do. The Constitution

preempts the City’s claims.

CONCLUSION

This Court should reverse.

Respectfully submitted,

STEVEN A. ENGEL

DECHERT LLP

1900 K Street, NW

Washington, DC 20006

MICHAEL H. MCGINLEY

Counsel of Record

BRIAN A. KULP

ANTHONY R. JADICK

DECHERT LLP

Cira Centre

2929 Arch Street

Philadelphia, PA 19104

(215) 994-2463

michael.mcginley@dechert.com

Counsel for Amicus Curiae

May 21, 2026