The Troops, Ships of War, Compact, and Invasion Clause

No State shall, without the Consent of Congress . . . keep Troops, or Ships of War in time of Peace, enter into any Agreement or Compact with another State, or with a foreign Power, or engage in War, unless actually invaded, or in such imminent Danger as will not admit of delay.

—Julian G. Ku

Introduction

Article I, Section 10, Clause 3 imposes four conditional constraints on the states: (i) States cannot impose any taxes on vessels based on their size, known as “duties of tonnage,” unless Congress consents (see Essay No. 85); (ii) states cannot “keep Troops, or Ships of War in time of Peace” unless Congress consents; (iii) states cannot “enter into any Agreement or Compact with another State, or with a foreign Power” unless Congress consents; and (iv) generally cannot “engage in War” unless Congress consents. However, a state can engage in war if it is “actually invaded, or in such imminent Danger as will not admit of delay.”

History Before 1787

The Articles of Confederation addressed the states’ powers to keep troops and ships of war, enter into certain alliances with other states, and engage in war if invaded. Article VI, Section 2 provided that states could enter into a “treaty, confederation, or alliance” only with the “consent of the united states, in congress assembled.” Before 1776, the Crown had to approve any intercolonial agreements.¹ The Articles substituted Congress for the Crown but changed past practice. Congressional consent was required only if two states made a “treaty, confederation, or alliance.” “In certain cases,” however, “the [authority] of the Confederacy was disregarded, as in violations not only of the treaty of Peace; but of Treaties with France & Holland, which were complained of to Cong[res]s.”² Federal authority was similarly violated by “Treaties & wars with Indians” and compacts made without the consent of Congress between Pennsylvania and New Jersey and between Virginia and Maryland.

Article VI, Section 4 prohibited states from keeping “vessels of war” in “time of peace” unless Congress deemed it “necessary” for “the defence of such state.” Section 4 also barred states from keeping “any body of forces . . . in time of peace” unless Congress consented. But the Articles also mandated that states “shall always keep up a well regulated disciplined militia, sufficiently armed and accoutred.” Thus, the legal regime that applied to a state military force depended on whether the force constituted an “army,” a “navy,” or a “militia.”

Under Article VI, Section 5, the Confederation Congress had the power to declare war. However, the states retained certain war powers. As a general matter, states could not “engage in any war without the consent” of Congress, but there were two exceptions. First, a state could engage in war if it “received certain advice of a resolution being formed by some nation of Indians to invade such State, and the danger is so imminent as not to admit of a delay till the united states in congress assembled, can be consulted.” Second, states could engage in war if “actually invaded by enemies,” regardless of whether there was enough time to consult Congress. If the danger was already ongoing, states could take immediate action. In short, Article VI, Section 5 “laid out a coherent scheme in which Congress received general authority to declare and wage war for the Confederation” and “[s]tate war powers were reserved but limited.”³ All three of these reserved state war powers would be retained in the new Constitution.

The Constitutional Convention

An early version of Article I, Section 10, Clause 3 appeared in a draft by Charles Pinckney of South Carolina that would have prohibited states from entering into alliances or “keeping troops or ships of war in time of peace.”⁴ By contrast, states could “engage in war” if “in self-defence, when actually invaded or the danger of invasion is so great as not to admit of a delay until the government of the United States can be informed thereof.”⁵ States would retain the power they had under the Articles of Confederation, to engage in war without consulting Congress in case of invasion. There was no temporal limitation on such powers in the event of an invasion.

The Committee of Detail’s draft made some modifications to Pinckney’s proposal. First, the proposed national legislature’s consent was required for states to enter into “any agreement or compact” with other states. This language was far broader than the terms of the Articles of Confederation, which applied only to a specific set of arrangements: a “treaty, confederation, or alliance.” Second, states could not “keep Troops or Ships of War in Time of Peace” without the national legislature’s consent. Third, “No State shall, without such Consent engage in any War, unless it shall be actually invaded by Enemies, or the Danger of Invasion be so imminent as not to admit of a Delay, until the Legislature of the United States can be consulted.”⁶

The Committee’s report, delivered to the Convention on August 6, tracked this draft.⁷ The text, two scholars observed, “divided war powers between the federal government and the states by granting authority to the federal government and limiting the reserved authority of the states.”⁸

On August 28, the delegates debated this provision but focused almost entirely on the states’ powers to enact imposts and duties.⁹ There was no discussion of troops and ships of war, compacts, and invasion. This text was referred to the Committee of Style, which made some minor changes.¹⁰ The Convention considered this provision on September 15, only two days before the Constitution would be signed.¹¹ The delegates discussed the Tonnage Clause, but there was still no debate about troops, ships of war, compacts, and invasion. “The remainder of the paragraph was then remoulded and passed as follows viz—‘No State shall without the consent of Congress, lay any duty of tonnage, keep troops or ships of war in time of peace, enter into any agreement or compact with another State, or with a foreign power, or engage in war, unless actually invaded, or in such imminent danger as will not admit of delay.’”¹² No further changes were made.

The Ratification Debates

The Invasion Clause in Article I, Section 10, Clause 3 recognizes the states’ reserved power to respond to an invasion. Tench Coxe, a Federalist essayist, wrote in the Pennsylvania Gazette that “[a]ny state may repel invasions or commence a war under emergent circumstances, without waiting for the consent of Congress.”¹³ Further, Article IV, Section 4 provides that the United States “guarantee[s] to every State . . . a Republican Form of Government” and “shall protect each [state] against invasion.” In Federalist No. 43, Madison explained that this “protection against invasion . . . seems to secure each state not only against foreign hostility, but against ambitious or vindictive enterprizes of its more powerful neighbours” (other states). (See Essay No. 147.) In Federalist No. 44, Madison wrote that the “reasonings” behind Article I, Section 10, Clause 3 “are either so obvious, or have been so fully developed, that they may be passed over without remark.”

During the Virginia ratifying convention, delegates discussed the risk of invasion. Madison said that “[a] republican government is to be guarantied to each state, and they are to be protected from invasion from other states, as well as from foreign powers.”¹⁴ Moreover, when states are “invaded, or in imminent danger . . . they are not restrained” in their response.¹⁵ Future Chief Justice John Marshall observed that “[a]ll the restraints intended to be laid on the state governments . . . are contained in” Article I, Section 10.¹⁶ He explained that this section does not prohibit states from “defend[ing] themselves without an application to Congress.”¹⁷ Rather, when states are “invaded, they can engage in war, as also when in imminent danger.”¹⁸ These two comments confirmed that states maintain a residual military power that can use to defend against invasions and against other imminent dangers.

The Compact Clause

The Compact Clause was rarely invoked in the first century after ratification and attracted little judicial attention. When states made agreements with each other, they usually received congressional consent.¹⁹ In the late nineteenth century, these sorts of agreements became more common. Virginia v. Tennessee (1893) held that congressional consent was needed only when an agreement would “encroach upon or impair the supremacy of the United States or interfere with their rightful management of particular subjects placed under their entire control.”²⁰ This interpretation allowed states to make agreements that did not threaten federal interests. Modern practice has favored a unitary practice that would subject foreign compacts to the same standard.²¹

The Supreme Court has never invalidated an interstate compact that lacked congressional consent, and the federal government has also taken a hands-off approach. States have tested the limits of the Compact Clause with increasingly elaborate interstate and international agreements. For instance, states entered into a multistate settlement for regulation of tobacco without federal approval.²² They also have made agreements with foreign states on subjects as varied as trade, natural resources management, drivers’ licenses, and climate change.²³ In 2019, the Trump Administration sued California to block the state’s agreement with Quebec on greenhouse gas emissions. A federal district court rejected the challenge, finding that the “Agreement does not contain the first indicium of a compact because it does not require reciprocal action to take effect.”²⁴

The Troops and Ships of War Clause

The Constitution recognizes three categories of military forces: armies, navies, and militia. Article I, Section 8, Clause 16 grants Congress the power to organize, arm, and discipline the militia (see Essay No. 62), but that provision also reserved to the states substantial power to govern and use the militia. States could conduct militia training, appoint militia officers, govern the militia when not in federal service, and use militia for domestic purposes.

The Framers took a more nationalistic approach to armies and navies. Article I, Section 10, Clause 3 banned states from “keep[ing] Troops, or Ships of War in time of Peace” without congressional consent.

Members of the Founding generation were not suspicious of the militia, which was a non-professional defensive force, but professional armies could project power outside a state’s borders and be used as instruments of domestic suppression. Members of the Founding generation were therefore very suspicious of them. Justice Joseph Story observed that the “setting on foot of an army, or navy, by a state in times of peace, might be a cause of jealousy between neighbouring states, and provoke the hostilities of foreign bordering nations.”²⁵

In Houston v. Moore (1820), Justice Story explained that since states were “expressly prohibited from keeping troops or ships of war in time of peace . . . the militia would be their natural and sufficient defence.”²⁶ How else, he asked, “could the States sufficiently provide for their own safety against domestic insurrections, or the sudden invasion of a foreign enemy?” United States v. Miller (1939) observed that “[t]he Militia which the States were expected to maintain and train is set in contrast with Troops which they were forbidden to keep without the consent of Congress.”²⁷

In the nineteenth century, states replaced their universal militia systems with organized volunteer units, often called the National Guard. The courts used professionalism as a criterion to distinguish between a permissible state militia and a prohibited state standing army with “Troops.” The Illinois Supreme Court, for example, explained that a militia is the “body of armed citizens trained to military duty, who may be called out in certain cases, but may not be kept on service like standing armies, in time of peace.”²⁸ Moreover, the term “troops” “conveys to the mind the idea of an armed body of soldiers, whose sole occupation is war or service.”²⁹ States retain concurrent power to organize their militia consisting of nonprofessional citizen soldiers, but if they want to raise professional troops in time of peace, they must seek congressional consent.

The proper constitutional classification of the naval militia poses additional difficulties. Federal law defines the organized militia in part to include the naval militia,³⁰ but whether the naval militia is constitutionally part of the militia is open to dispute. On the one hand, because a naval militia requires armed ships, it may be a separate state navy under Article I, Section 10.³¹ Alternatively, the term “Ships of War” may include only armed vessels capable of projecting power abroad as distinguished from small armed vessels of a purely defensive nature.

What Is an Invasion?

The Supreme Court has recognized that under the Guarantee Clause, the federal government is charged with “protect[ing] each” state “against Invasion.”³² Justice Antonin Scalia observed that Article I, Section 10, Clause 3 serves as an “acknowledgment of the States’ sovereign interest in protecting their borders” and “leaves intact [States’] inherent power to protect their territory.”³³ The Supreme Court has not defined what constitutes an invasion, but several lower courts have interpreted “invasion” to mean an incursion by a foreign army.³⁴

Some originalist scholars adopt a more expansive definition of “invasion.” Robert Natelson and Andrew Hyman contend that invaders do not need to be organized, armed, or part of a foreign army. They point out that during the Founding era, “Pennsylvanians used [the] term [invasion] to describe the essentially peaceful immigration of Connecticut settlers into Pennsylvania’s Wyoming Valley, because the settlers were relying on legal title that the Pennsylvania government did not recognize.”³⁵ They conclude that “[a]n incursion qualifies as an invasion if it is unauthorized and uninvited and causes or threatens detriment beyond the mere fact of crossing.”³⁶ Ilya Somin, on the other hand, argues that the term “invasion” refers only to “large-scale use of force (or at least threat of force) to seize territory.”³⁷ Somin relies on James Madison’s Report of 1800, authored for the Virginia legislature, which specifies that “invasion is an operation of war.”³⁸

Who Can Determine Whether There Is an Invasion?

Article IV, Section 4 provides that the United States “shall protect each [state] against Invasion.” But if a state is “actually invaded,” it can “engage in war” without obtaining congressional consent. Who can determine whether there is an invasion? Congress has the sole power to “declare war.” It also has the power to “suspend[]” the “Privilege of the Writ of Habeas Corpus . . . in cases of Invasion.”³⁹ (See Essay No. 68.) This text suggests that Congress likewise has the power to declare that there is an invasion.

Congress can establish the rules by which the President, as commander in chief of the militia, can “repel Invasions.”⁴⁰ Here the text suggests that the President can determine in the first instance when an invasion is occurring. President Franklin D. Roosevelt referred to the attack on Pearl Harbor as a “premeditated invasion.”⁴¹ In 2025, President Donald Trump declared an invasion at the southern border.⁴²

It is an open question whether a state executive can also declare an invasion, which would trigger the state’s war powers. In 1874, Texas Governor Richard Coke declared an invasion. He invoked the state’s “inherent right of self-defense” and “the war powers that would ‘ordinarily reside in the United States.’”⁴³ In 2024, Texas Governor Greg Abbott declared an invasion along the southern border to justify installing barriers in the Rio Grande River.⁴⁴

The judiciary has ruled that certain political questions cannot be resolved by the courts but instead must be settled by the elected branches. The courts have generally found that whether a President’s use of military force requires a declaration of war is a nonjusticiable political question in the absence of “a coherent test for judges to apply to the question what constitutes war. . . .”⁴⁵ The Court also deferred to a governor’s finding that there is an insurrection.⁴⁶

The determination as to whether there is an invasion also appears to be a political question. A court of appeals has ruled that “there are no manageable standards to ascertain whether or when an influx of illegal immigrants should be said to constitute an invasion.”⁴⁷ One judge has found that “[d]etermining where the present illegal immigration crisis falls along this spectrum [of invasions] is not a legal question for judges, but a political determination for the other branches of government.”⁴⁸

When Can a State Engage in War?

The text provides that “No State shall, without the Consent of Congress . . . engage in War, unless actually invaded, or in such imminent Danger as will not admit of delay.” In other words, when a delay in engaging in war is feasible, the state must obtain congressional consent to “engage in war” against an “imminent danger.” As a textual matter, does the phrase “will not admit of delay” also modify “actually invaded”? In United States v. Abbott, the federal government argued “that any right that a State may have to defend itself [against an invasion] is a ‘time-limited emergency authority,’ and that the right ceases once the federal government has had the opportunity to respond.”⁴⁹ Although the Abbott court did not rule on this argument, one judge opined that a state can “engage in war” to stop an actual invasion immediately with “no such temporal restriction.”⁵⁰

Open Questions

• After congressional approval, must a foreign or interstate compact be presented to the President under Article I, Section 7, Clause 2? As a matter of practice, such compacts have been presented to the President in the same manner as ordinary legislation,⁵¹ and the threat of a presidential veto has stalled compacts.⁵² However, it is possible that an interstate compact could be approved through a concurrent resolution without presidential presentment under Article I, Section 7, Clause 3.⁵³

1. Felix Frankfurter & James M. Landis, The Compact Clause of the Constitution—A Study in Interstate Adjustments, 34 Yale L.J. 685, 692, 730–32 (1925). ↩︎
2. 3 Farrand’s 548; Duncan B. Hollis, Unpacking the Compact Clause, 88 Tex. L. Rev. 741, 760 (2010). ↩︎
3. Robert G. Natelson & Andrew T. Hyman, The Constitution, Invasion, Immigration, and the War Powers of States, 13 Br. J. Am. Legal Studies 1, 13 (2024). ↩︎
4. 1 Elliot’s 149. ↩︎
5. Id. ↩︎
6. 2 Farrand’s 169. ↩︎
7. Id. at 187. ↩︎
8. Natelson & Hyman, supra at 14. ↩︎
9. 2 Farrand’s 442. ↩︎
10. Id. at 565, 570, 597. ↩︎
11. Id. at 625. ↩︎
12. Id. at 626. ↩︎
13. 15 DHRC 510. ↩︎
14. 3 Elliot’s 425. ↩︎
15. Id. ↩︎
16. Id. at 420. ↩︎
17. Id. at 419. ↩︎
18. Id. at 420. ↩︎
19. Frankfurter & Landis, supra at 735–54. ↩︎
20. Virginia v. Tennessee, 148 U.S. 503, 518 (1893). ↩︎
21. Letter from William H. Taft, IV, Legal Adviser, Dep’t of State, to Senator Byron L. Dorgan of North Dakota Regarding a Memorandum of Understanding Signed by the State of Missouri and the Province of Manitoba (Nov. 20, 2001), https://perma.cc/Z895-LRQ4. ↩︎
22. The Master Settlement Agreement, https://perma.cc/VR9K-TKTX. ↩︎
23. Julian G. Ku, Gubernatorial Foreign Policy, 115 Yale L.J. 2380, 2392–98 (2006). ↩︎
24. United States v. California, 444 F. Supp. 3d 1181, 1194 (E.D. Cal. 2020). ↩︎
25. 3 Story’s Commentaries § 1398. ↩︎
26. Houston v. Moore, 18 U.S. 1, 52 (1820). ↩︎
27. United States v. Miller, 307 U.S. 174, 178–79 (1939). ↩︎
28. Dunne v. People, 94 Ill. 120, 138 (1879). ↩︎
29. Id. at 138–39. ↩︎
30. 10 U.S.C. § 246(b)(1). ↩︎
31. Robert Leider, Federalism and the Military Power of the United States, 73 Vand. L. Rev. 989, 1001 n.56 (2020). ↩︎
32. Torres v. Texas Dep’t of Pub. Safety, 597 U.S. 580, 590 (2022). ↩︎
33. Arizona v. United States, 567 U.S. 387, 419 (2012) (Scalia, J., concurring in part and dissenting in part). ↩︎
34. California v. United States, 104 F.3d 1086, 1091 (9th Cir. 1997); Padavan v. United States, 83 F.3d 23, 28 (2nd Cir. 1996); New Jersey v. United States, 91 F.3d 463, 468 (3rd Cir. 1996). ↩︎
35. Natelson & Hyman, supra at 23–24. ↩︎
36. Id. at 36. ↩︎
37. Ilya Somin, Immigration Is Not “Invasion,” Volokh Conspiracy (May 18, 2023), https://perma.cc/GR6C-GBV8. ↩︎
38. James Madison, The Report of 1800 (Jan. 7, 1800), https://perma.cc/SYU5-BWY2. ↩︎
39. Art. I, § 9, cl. 2. ↩︎
40. Art. I, § 8, cl. 14. ↩︎
41. Franklin D. Roosevelt, Joint Address to Congress Leading to a Declaration of War Against Japan (Dec. 8, 1941), https://perma.cc/BE3K-QHKM. ↩︎
42. President Donald J. Trump, Proclamation Guaranteeing the States Protection Against Invasion (Jan. 20, 2025), https://perma.cc/W54Z-XUGM. ↩︎
43. United States v. Abbott, 110 F.4th 700, 726, 743–44 (5th Cir. 2024) (Ho, J., concurring in part and dissenting in part) (quoting Texas Frontier Troubles, H.R. Rep. No. 44-343, at 166–67 (1876)). ↩︎
44. Governor Greg Abbott, Statement on Constitutional Right of Texas to Self-Defense (Jan. 24, 2024), https://perma.cc/EQM9-XDVF. ↩︎
45. Campbell v. Clinton, 203 F.3d 19, 26 (D.C. Cir. 2000) (Silberman, J., concurring). ↩︎
46. Sterling v. Constantin, 287 U.S. 378, 399 (1932). ↩︎
47. California v. United States, 104 F.3d 1086, 1091 (9th Cir. 1997). ↩︎
48. Abbott, 110 F.4th at 728 (Ho, J.). ↩︎
49. Id. at 730. ↩︎
50. Id. at 725 n.1 (Ho, J.). ↩︎
51. Bradford R. Clark, Domesticating Sole Executive Agreements, 93 Va. L. Rev. 1573, 1661 (2007); Edward T. Swaine, Does Federalism Constrain the Treaty Power?, 103 Colum. L. Rev. 403, 508 (2003). ↩︎
52. Dana Brakman Reiser, Note, Charting No Man’s Land: Applying Jurisdictional and Choice of Law Doctrines to Interstate Compacts, 111 Harv. L. Rev. 1991, 2008 (1998). ↩︎
53. Josh Blackman, Does the Compact Clause Require Presentment?. Josh Blackman’s Blog (Nov. 16, 2014), https://perma.cc/7AR5-EWQQ. ↩︎