Few constitutional issues have been so consistently and heatedly debated by legal scholars and politicians in recent years as the distribution of war powers between Congress and the President. As a matter of history and policy, it is generally accepted that the executive takes the lead in the actual conduct of war. A single, energetic actor is better able to prosecute war successfully than a committee; the enemy will not wait for deliberation and consensus. At the same time, the Founders plainly intended to establish congressional checks on the executive’s war power. Between these guideposts is a question of considerable importance: Does the Constitution require the President to obtain specific authorization from Congress before initiating hostilities?

Presidential power advocates argue that Article II, Section 1 (which vests the “executive Power” in the President of the United States) and Article II, Section 2 (which designates the President as “Commander in Chief of the Army and Navy of the United States . . .”) confer substantive constitutional power upon the President to initiate military hostilities with foreign nations. The leading political thinkers to influence the Framers, such as Machiavelli, Thomas Hobbes, John Locke, William Blackstone, and Montesquieu, understood the executive power to arise from the need for a part of government to act quickly, vigorously, and decisively in response to unforeseen dangers and events—the most serious of which was war. Executives throughout British history as well as in the colonial governments and several of the states prior to the Constitution generally enjoyed such power.

The Declare War Clause, in this view, was not addressed to the power to begin actual hostilities. Instead, declarations of war altered legal relationships between subjects of warring nations and triggered certain rights, privileges, and protections under the laws of war. According to Hugo Grotius, declarations gave notice of the legal grounds for the war and the opportunity for enemy nations to make amends. They served notice on the enemy’s allies that they would be regarded as cobelligerents and their shipping would be subject to capture. Under a declaration of war, one’s own navy and privateers could not be treated as pirates by the enemy, but on the other hand one’s own citizens were subject to prosecution if they dealt with the enemy. Additionally, declarations of war triggered legal actions, such as the internment or expulsion of enemy aliens, the breaking of diplomatic relations, and the confiscation of the enemy’s property. In short, in this view, the power to declare war was understood as a power to affect legal rights and duties in times of hostilities. It was not a check on executive power to engage in hostilities in the first place.

The check on executive war power, it is argued, instead comes from Congress’s power over appropriations. Any power to initiate hostilities would be useless, of course, without the resources necessary to engage in hostilities. Under our Constitution, the power to provide those resources is unequivocally vested with Congress. Under Article I, it is Congress, not the President, that has the power to “lay and collect Taxes” and to “borrow Money,” to make “Appropriations” and “provide for the common Defence,” to “raise and support Armies” and “provide and maintain a Navy,” and to “provide for calling forth the Militia.” The President is Commander in Chief, but he has nothing to command except what Congress provides. As a result of Congress’s authority over the purse, the President is unable as a practical (if not constitutional) matter to engage in hostilities alone. Based on these provisions of the Constitution, some scholars have concluded that Congress’s war power is limited to its control over funding and its power to impeach executive officers. They contend that the President is constitutionally empowered to engage in hostilities with whatever resources Congress has made available to the executive.

In contrast, many scholars contend that the Declare War Clause limits presidential war power by giving the legislature the sole authority to begin an offensive war. In one reading, the clause requires Congress to issue a formal declaration of war before the United States may begin hostilities. The more common view among these scholars, however, is that the clause requires congressional authorization of hostilities, which may be done by formal declaration or otherwise. The latter interpretation is often textually grounded in the observation that in eighteenth-century terminology “declaring” could be done by a formal proclamation or simply by initiating hostilities. As Emmerich de Vattel wrote in 1758, “when one nation takes up arms against another, she from that moment declares herself an enemy to all the individuals of the latter.” Thus the Declare War Clause is read to give Congress power to initiate war either by issuing a formal declaration or by directing actions (such as hostile attacks) that have the effect of signaling—declaring informally—the start of a war.

Advocates of congressional power also point to numerous statements during and after the drafting process saying or heavily implying that the President alone cannot initiate war. For example, James Madison wrote to Thomas Jefferson in 1798, “The constitution supposes, what the History of all Governments demonstrates, that the Executive is the branch of power most interested in war, and most prone to it. It has accordingly with studied care, vested the question of war in the Legislature.” Alexander Hamilton, though generally a strong proponent of presidential power, wrote in his 1793 Pacificus essay: “It is the province and duty of the executive to preserve to the nation the blessings of peace. The Legislature alone can interrupt those blessings by placing the nation in a state of war.” Defenders of presidential power discount these post-ratification statements and respond that the records of the ratification debates show no Federalists or Anti-Federalists identifying the clause as a significant check on executive activity. If anything, they say, the debates looked to Congress’s power over the establishment and funding of the military as the critical legislative tool for restraining the executive.

Advocates of presidential power further argue that the text distinguishes between “declaring” and beginning war. Article I, Section 10, Clause 3 forbids states from “engaging in” war without the consent of Congress. Article III, Section 3, Clause 1 defines treason as “levying” war. The Articles of Confederation also suggested that declaring war is but a subset of the broader power over war. Article IX of the Articles gave to Congress “the sole and exclusive right and power of determining on peace and war,” and Article VI similarly declared that “no state shall engage in any war” without “the consent of Congress.” If “declare” war held the meaning attributed to it by supporters of congressional power, it is argued, the Framers should have used the same phrases in these other constitutional provisions instead of “engage” or “levy.”

History casts some doubt on the idea that the Framers would have thought a formal declaration was prerequisite to initiating armed conflict. In the one hundred years preceding the ratification of the Constitution, the British rarely declared war formally before the outbreak of hostilities. The Founders, who drew the language in the Declare War Clause directly from British constitutional practice (as they were British citizens for much of their lives), were well aware of the long practice of undeclared wars, as Hamilton noted in The Federalist No. 25: “the ceremony of a formal denunciation of war has of late fallen into disuse.” They also would have been well aware of the nation’s first declaration of war—the Declaration of Independence—which issued a year after hostilities broke out between the colonists and the British at Lexington and Concord. Since the ratification of the Constitution, there have been only five wars formally declared by Congress in the history of the United States. Of those, only the first, the War of 1812, was formally declared before the start of hostilities. In the remaining four, the Mexican-American War of 1846, the Spanish-American War of 1898, World War I, and World War II, Congress merely declared the prior existence of a state of war. Notably, those declarations were accompanied by express authorizations of the use of force, suggesting a distinction in practice between the two.

Congress has, however, specifically authorized numerous other conflicts using instruments other than formal declarations of war; and particularly in the early post-ratification period there appeared to have been broad consensus that such authorizations were constitutionally required in the case of offensive war. For example, offensive actions taken by the United States during its first real “war”—against Tripoli beginning in 1801— were statutorily authorized but not accompanied by a formal declaration. Although the President arguably directed some offensive action against Tripoli prior to congressional authorization, this was defended (including by Hamilton) only on the ground that Tripoli had itself begun the war. Congress also expressly authorized the use of force in the Quasi-War with France in 1798, and it was assumed at the time that such authorization was needed.

In much of American history, limited budgets and a small peacetime military led Presidents to defer to Congress. If Presidents wanted to wage a war, they had to ask Congress to build them the armed forces with which to fight it. After World War II, however, the creation of a large standing military dramatically altered the balance of war powers between the two branches. Beginning with Franklin D. Roosevelt, modern Presidents have been more aggressive in asserting unilateral authority to use force abroad without a declaration of war or other congressional authorization. The United States intervention in Korea in 1950 received congressional support but no formal approval. When the war stalemated, executive power was challenged. President Harry S. Truman responded by claiming independent constitutional authority to commit troops without congressional authorization. Presidents Lyndon B. Johnson and Richard M. Nixon undertook military operations in Vietnam, armed with only the vaguely worded congressional approval in the Gulf of Tonkin Resolution. Congressional criticism of that protracted campaign led not only to funding restrictions that forced the end of U.S. participation in the war, but also to the 1973 enactment of the War Powers Resolution, over President Nixon’s veto. The Resolution claims to limit the President’s ability to engage U.S. forces in hostilities for more than sixty days, absent a declaration of war or specific congressional authorization, and requires the President to consult with Congress about military deployments.

The War Powers Resolution has proven impotent. No President has ever conceded its constitutionality, though many have acted “consistent with” its 60-day limit on foreign interventions. President James Earl Carter did not consult with Congress before attempting to rescue Iranian hostages. President Ronald Reagan unilaterally dispatched American military forces to Lebanon, Grenada, Libya, and the Persian Gulf. Before Desert Storm, President George H.W. Bush publicly declared that he had constitutional power to initiate war unilaterally, but sought an authorization from Congress, which he barely received. President William Jefferson Clinton followed these precedents in Somalia, Haiti, Bosnia, the Middle East, and Kosovo. Ironically, given the attacks on his exercise of executive power, President George W. Bush asked for and received congressional approval for wars in Afghanistan and Iraq. But President Barack Obama unilaterally ordered a limited intervention in the Libyan civil war in 2011, on the ground that the intervention did not involve sustained hostilities, did not include ground troops, was part of a multilateral coalition to implement a U.N. Security Council resolution, and the hostilities were too small to constitute a “war” under the Constitution.

Members of Congress have periodically filed suit to enforce the War Powers Resolution and the congressionalist interpretation of the Declare War Clause, but courts have generally avoided ruling on the merits by dismissing such cases on a variety of procedural grounds. In Campbell v. Clinton (2000), for example, the D.C. Circuit unanimously dismissed a congressional challenge to President Clinton’s airstrikes campaign in the former Yugoslavia,under a panoply of competing theories arising out of the legislative standing, mootness, and political question doctrines. In O’Connor v. United States (2003), the court dismissed a challenge to President George W. Bush’s intention behind the war in Iraq because it posed a nonjusticiable political question and “there are no judicially discoverable standards that would permit a court to determine whether the intentions of the President in prosecuting a war are proper.”

The Supreme Court has never intervened to stop a war, regardless of whether Congress had authorized it, although it did say (in dicta) in The Prize Cases (1863) that the President “has no power to initiate or declare a war.” Some federal courts of appeals have held or implied that at least some level of congressional authorization is constitutionally required before the President may conduct military hostilities. See, e.g., Orlando v. Laird (1971). Other courts have found the issue nonjusticiable. See, e.g., Mitchell v. Laird (1973).

Even if the Declare War Clause is thought to limit the President’s ability to initiate war, almost all scholars accept that it does not prevent the President from defending against attacks upon the United States. James Madison successfully advocated in the Constitutional Convention that Congress be given the power, not to “make” war but to “declare” war, so as to (as he said) “leav[e] to the Executive the power to repel sudden attacks.” In The Prize Cases, the Supreme Court upheld President Abraham Lincoln’s military actions against the Confederacy, although they occurred prior to congressional authorization, because “the President is not only authorized but bound to resist force by force . . . without waiting for any special legislative authority.”

The scope of the President’s defensive power remains debated, however. One key question is whether, once an attack on the United States occurs, the President may authorize counter-attacks against the enemy without Congress’s approval. Alexander Hamilton, for example, took this view in 1801 in connection with hostilities against Tripoli, and in The Prize Cases the Supreme Court approved President Lincoln’s unauthorized blockade of the southern states, which was not purely a defensive measure. The opposing view is that the President may only act defensively; offensive action, even in response to an attack, requires congressional approval under the Declare War Clause. Relatedly, there is debate over whether and to what extent the President may act against an anticipated attack without Congress’s approval.

Whatever the domestic constitutional implications for the initiation of hostilities, the Declare War Clause gives Congress certain powers under international and domestic statutory law. Formally declaring war defines the legal relationship between the citizens of the United States and that of the enemy nation. It also expands the powers of the U.S. government vis-à-vis its own citizens at home. A particularly dramatic example is the Alien Enemies Act (1 Stat. 577 (1798), codified in 50 U.S.C. § 21 (2003)), which authorizes the President to detain and deport citizens of enemy nations, but only following either a declaration of war or an attack upon the United States. Nonetheless, the legal significance of formal declarations has declined. For example, the Geneva Conventions of 1949, which guarantee various enumerated rights to lawful combatants, prisoners of war, and civilians, explicitly apply to all armed conflicts between contracting nations and not just to formally declared wars.