A declaration of war is a formal declaration issued by a national government indicating that a state of war exists between that nation and another. A document by the Federation of American Scientists gives an extensive listing and summary of statutes which are automatically engaged upon the United States declaring war.^[1]

For the United States, Article One, Section Eight of the Constitution says "Congress shall have power to ... declare War." However, that passage provides no specific format for what form legislation must have in order to be considered a "declaration of war" nor does the Constitution itself use this term. In the courts, the United States Court of Appeals for the First Circuit, in Doe v. Bush, said: "[T]he text of the October Resolution itself spells out justifications for a war and frames itself as an 'authorization' of such a war."^[2] in effect saying an authorization suffices for declaration and what some may view as a formal congressional "Declaration of War" was not required by the Constitution.

The last time the United States formally declared war, using specific terminology, on any nation was in 1942, when war was declared against Axis-aligned Hungary, Bulgaria, and Romania, because President Franklin Roosevelt thought it was improper to engage in hostilities against a country without a formal declaration of war. Since then, every American president has used military force without a declaration of war.^[3]

This article will use the term "formal declaration of war" to mean congressional legislation that uses the phrase "declaration of war" in the title. Elsewhere, this article will use the terms "authorized by Congress," "funded by Congress" or "undeclared war" to describe other such conflicts.

The United States has formally declared war against foreign nations in five separate wars, each upon prior request by the president of the United States. Four of those five declarations came after hostilities had begun.^[4] James Madison reported that in the Federal Convention of 1787, the phrase "make war" was changed to "declare war" in order to leave to the executive the power to repel sudden attacks but not to commence war without the explicit approval of Congress.^[5] Debate continues as to the legal extent of the president's authority in this regard.

Public opposition to American involvement in foreign wars, particularly during the 1930s, was expressed as support for a Constitutional Amendment that would require a national referendum on a declaration of war.^[6] Several constitutional amendments, such as the Ludlow Amendment, have been proposed that would require a national referendum on a declaration of war.

After Congress repealed the Gulf of Tonkin Resolution in January 1971 and President Richard Nixon continued to wage war in Vietnam, Congress passed the War Powers Resolution (Pub. L. 93–148) over the veto of Nixon in an attempt to rein in some of the president's claimed powers. The War Powers Resolution proscribes the only power of the president to wage war which is recognized by Congress.^[7]

The table below lists the five wars in which the United States has formally declared war against ten foreign nations.^[8] The only country against which the United States has declared war more than once is Germany, against which the United States has declared war twice (though a case could be made for Hungary as a successor state to Austria-Hungary).

In World War II, the Japanese attacked Pearl Harbor on December 7, 1941. Germany and Italy, led respectively by Adolf Hitler and Benito Mussolini, declared war on the United States, and the U.S. Congress responded in kind.^[9][10]

War	Declaration	Opponent(s)	Date of declaration	Votes		President	Details of Authorization	Result
				Senate	House
War of 1812	Declaration of war on the United Kingdom	United Kingdom and its dependencies[11][12]	June 18, 1812	19–13	79–49	James Madison	Madison requested the declaration citing the Royal Navy's impressment of U.S. citizens and other alleged violations of American neutrality during the Napoleonic Wars. The declaration was supported by the Democratic-Republican Party and other war hawks in Congress and opposed by the Federalist Party.[12][13]	Treaty of Ghent (December 24, 1814)
Mexican–American War	Declaration of war on Mexico[14]	Mexico	May 13, 1846	40–2	173–14	James K. Polk	Polk requested the declaration citing territorial disputes with the Mexican government, which refused to recognize the Rio Grande as the Mexico–United States border after the U.S. annexation of Texas. Mexican president José Joaquín de Herrera refused to enter negotiations with U.S. ambassador John Slidell, and the Mexican Army attacked the United States Army on disputed territory in the 1846 Thornton Affair. The declaration was opposed by the Whig Party, which viewed the war as a pretext for the Democratic Party to incorporate more slave states into the United States.[12][15]	Treaty of Guadalupe Hidalgo (February 2, 1848)
Spanish–American War	Declaration of war on Spain	Spain	April 25, 1898	42–35	310–6	William McKinley	McKinley requested the declaration citing the sinking of the USS Maine in Havana Harbor after years of declining Spain–United States relations as a result of the Cuban War of Independence. Unlike other declarations, it occurred after the U.S. issued an ultimatum demanding the Spanish Empire grant independence to Cuba.[16]	Treaty of Paris (December 10, 1898)
World War I	Declaration of war on Germany	Germany	April 6, 1917	82–6	373–50	Woodrow Wilson	Wilson requested the declaration citing the Imperial German Navy's resumption of unrestricted submarine warfare and the Zimmermann Telegram offering Mexico a return of the Mexican Cession if it allied with the Central Powers and invaded the United States.[12][16]	Knox–Porter Resolution (July 1921) 1921 U.S.–German Peace Treaty (August 25, 1921)
	Declaration of war on Austria-Hungary[17][18]	Austria-Hungary	December 7, 1917	74–0	365–1		Wilson requested the declaration citing Austria-Hungary's alliance with Germany and participation in attacks on U.S. citizens, including the Austro-Hungarian Navy's complicity in submarine attacks on American shipping.[12][16]	1921 U.S.–Austrian Peace Treaty (August 24, 1921), 1921 U.S.–Hungarian Peace Treaty (August 29, 1921)
World War II	Declaration of war on Japan	Japan	December 8, 1941	82–0	388–1	Franklin D. Roosevelt	Roosevelt requested the declaration citing the Imperial Japanese Armed Forces' attack on the United States Pacific Fleet in the attack on Pearl Harbor a day earlier. It was approved near-unanimously with only one dissenting vote in the entire Congress from Jeannette Rankin.[12][19]	Surrender of Japan V-J Day, Japanese Instrument of Surrender (September 2, 1945), Treaty of San Francisco (September 8, 1951)
	Declaration of war on Germany	Germany	December 11, 1941	88–0	393–0		Roosevelt requested the declaration in response to the German declaration of war against the United States. Approved unanimously with no dissenting votes.[12][19]	German Instrument of Surrender (May 8, 1945), V-E Day, Public Law 181 (October 19, 1951), Treaty on the Final Settlement with Respect to Germany (September 12, 1990), Treaty of Vienna with Austria (May 15, 1955)
	Declaration of war on Italy	Italy		90–0	399–0		Roosevelt requested the declaration in response to the Italian declaration of war on the United States. Approved unanimously with no dissenting votes.[12][19]	Armistice of Cassibile (September 3, 1943) Treaty of Peace with Italy, 1947 Paris Peace Treaties (February 10, 1947)
	Declaration of war on Bulgaria	Bulgaria	June 5, 1942	73–0	357–0		Roosevelt requested the declaration citing the three countries' involvement in the Axis powers with Germany and Italy.[12][19]
	Declaration of war on Hungary[17][20]	Hungary			360–0
	Declaration of war on Romania[17][21]	Romania			361–0

In other instances, the United States has engaged in extended military combat that was authorized by Congress.

War or conflict	Opponent(s)	Initial authorization	Votes		President	Details of Authorization	Result
			Senate	House
Quasi-War	France	An Act further to protect the commerce of the United States July 9, 1798	18–4		John Adams	Adams requested legislation allowing the United States Navy to defend American shipping after repeated attacks by the French Navy during the French Revolutionary Wars.[22]	Treaty of Mortefontaine
First Barbary War	Morocco Tripolitania	"An Act for the Protection of the Commerce and Seamen of the United States, Against the Tripolitan Cruisers", 2 Stat. 129, February 6, 1802[23]			Thomas Jefferson	President Jefferson requested legislation allowing the U. S. Navy to defend shipping in the Mediterranean Sea from Tripolitanian vessels.[24]	War ended 1805
Second Barbary War	Algiers	"An Act for the protection of the commerce of the United States against the Algerine cruisers", 3 Stat. 230, May 10, 1815[25]			James Madison	Madison requested a declaration of war against Algiers citing attacks on U.S. shipping in the Mediterranean. Congress rejected the request for a formal war declaration but ratified legislation allowing the U.S. Navy to defend U.S. commerce.[24]	War ended 1816
Enforcing 1808 slave trade ban; naval squadron sent to African waters to apprehend illegal slave traders	Slave traders	"Act in addition to the acts prohibiting the Slave Trade", 3 Stat. 532, 1819			James Monroe		1822 first African-American settlement founded in Liberia, 1823 U.S. Navy stops anti-trafficking patrols
Suppression of Piracy	Pirates	1819			James Monroe	The United States Congress passed legislation allowing the United States Navy to suppress piracy in response to the rise in piracy in Latin America and the Caribbean after the Spanish American wars of independence, which was later permanently codified as Title 33 of the United States Code.[22]
Redress for attack on U.S. Navy's USS Water Witch	Paraguay	1858.[26]			James Buchanan
Mexican Revolution United States occupation of Veracruz	Mexico	H.J.R. 251, 38 Stat. 770 April 22, 1914[27]		337–37	Woodrow Wilson		Force withdrawn after six months. However, the Joint Resolution was likely used to authorize the Pancho Villa Expedition. In the Senate, "when word reached the Senate that the invasion had gone forward before the use-of-force resolution had been approved, Republicans reacted angrily" saying it was a violation of the Constitution, but eventually after the action had already started, a resolution was passed after the action to "justify" it since Senators did not think it was a declaration of war.[28][29]
Russian Civil War Allied intervention in the Russian Civil War	Commune of Estonia Far Eastern Republic Latvia Mongolian People's Party Russia Ukraine	1918[30]			Woodrow Wilson
Lebanon crisis of 1958	Lebanese Opposition Al-Mourabitoun Lebanese Communist Party Progressive Socialist Party	H.J. Res. 117, Public Law 85-7, Joint Resolution "To promote peace and stability in the Middle East", March 9, 1957[31]	72–19	355–61	Dwight D. Eisenhower	Eisenhower requested a legislation allowing U.S. economic and military assistance to the Middle East during the Cold War, including the ability to deploy the military in response to threatened Communist takeovers.[32]	U.S. forces withdrawn, October 25, 1958
Vietnam War Laotian Civil War Cambodian Civil War	Mainland China National United Front of Kampuchea Khmer Rouge Khmer Rumdo Khmer Việt Minh  North Korea North Vietnam Pathet Lao South Vietnam Việt Cộng	Gulf of Tonkin Resolution, August 7, 1964	88–2	416–0	Lyndon B. Johnson	Johnson requested authorization for a military deployment to defend South Vietnam and U.S. military forces already stationed there from under SEATO collective security obligations, citing alleged Vietnam People's Navy attacks on United States Navy warcraft including the USS Maddox in the Gulf of Tonkin incident. Congress responded with the Gulf of Tonkin Resolution.[33]	U.S. forces withdrawn under terms of the Paris Peace Accords signed January 27, 1973
Multinational Force in Lebanon	Shia militias, Druze militias,  Syria	S.J.Res. 159 Pub. L. 98–119 September 29, 1983	54–46	253–156	Ronald W. Reagan	Reagan announced the deployment of a small United States Marine Corps contingent of forces for peacekeeping in the Lebanese Civil War, claiming they would supervise the PLO withdrawal from Beirut and provide law enforcement, but not participate in direct combat. After Congress invoked the War Powers Resolution it and the Reagan administration negotiated a resolution allowing the marines to remain in Lebanon for 18 months.[34]
Persian Gulf War	Ba'athist Iraq	H.J.Res. 77 January 12, 1991.	52–47	250–183	George H.W. Bush	Bush announced the deployment of 330,000 United States Armed Forces troops to Saudi Arabia in response to the Iraqi invasion of Kuwait and began diplomatic overtures to form an international coalition to defend the Arab states of the Persian Gulf. Congress approved an Authorization for Use of Military Force against Ba'athist Iraq to liberate Kuwait under United Nations Security Council Resolution 678.[35]	The United Nations Security Council drew up terms for the cease-fire, April 3, 1991. The administration of George W. Bush later argued that the AUMF never expired during the build-up to the Iraq War. The United States House of Representatives voted to repeal it in 2021.[36]
War on Terror	Afghanistan Quetta Shura Taliban Haqqani network Mullah Dadullah Front al-Qaeda 055 Brigade Al-Nusra Front Khorasan group al-Qaeda Emirate in Yemen Aden-Abyan Islamic Army Islamic Jihad of Yemen al-Qaeda in the Arabian Peninsula Ansar al-Sharia al-Qaeda in the Indian Subcontinent Lashkar al-Zil Harakat Ahrar al-Sham al-Islamiyya Hezb-e Islami Gulbuddin Islamic Jihad Union Islamic Movement of Uzbekistan Jamaat-ul-Ahrar Jundallah Lashkar-e-Islam Lashkar-e-Jhangvi Tehreek-e-Nafaz-e-Shariat-e-Mohammadi Turkistan Islamic Party Tehrik-i-Taliban Pakistan High Council of the Islamic Emirate Fidai Mahaz al-Itihaad al-Islamiya Alliance for the Re-liberation of Somalia Harakat al-Shabaab Mujahedeen Hizbul Islam Islamic Courts Union Jabhatul Islamiya Mu'askar Anole Ras Kamboni Brigades Abu Sayyaf Bangsamoro Islamic Freedom Fighters Islamic State Islamic Movement of Uzbekistan Maute group Khalifa Islamiyah Mindanao	S.J. Res. 23 September 14, 2001	98–0	420–1	George W. Bush	Bush successfully requested a congressional authorizing the president of the United States to use military force against "those nations, organizations, or persons he determines planned, authorized, committed, or aided the terrorist attacks that occurred on September 11, 2001" as well as governments which sheltered them such as the First Islamic Emirate of Afghanistan. It also allowed the president to use force to prevent future acts of terrorism. Since then the authorization has been invoked in conflicts in 22 countries against the original perpetrator of 9/11 al-Qaeda as well as other organizations such as Al-Shabaab, the Taliban, and the Islamic State. The authorization is also notable in that it delegated war powers related to terrorism from Congress to the president, and allowed the United States to make war against individuals and organizations in addition to sovereign states. The Supreme Court ruled in Hamdi v. Rumsfeld that the authorization enabled the president to detain individuals, including U.S. citizens, as enemy combatants, although it granted detainees to challenge this status in U.S. courts and further ruled in Hamdan v. Rumsfeld that they were protected by laws of war such as the Geneva Conventions and the Uniform Code of Military Justice.[37][38][39][40][41]	The Global War on Terror is ongoing. The War in Afghanistan (2001–2021), that was carried out by the United States under the Global War on Terror's general authorization for use of military force, came to an end on August 30, 2021 with the total withdrawal of the American Forces from Afghanistan under the terms of the Doha Peace Agreement signed on February 29, 2020. The U.S. disengagement from Afghanistan resulted in the Fall of Kabul to the Taliban on August 15, 2021 and in a broad re-establishment of the status quo ante bellum. The U.S. backed Islamic Republic of Afghanistan collapsed even before the completion of the American withdrawal, and the Taliban victory led to the restoration of the Islamic Emirate of Afghanistan. Other U.S. military campaigns that are legally based on the Global War on Terror's general authorization for use of military force include the ongoing American-led intervention in the Syrian civil war that was initiated on September 22, 2014 under President Barack Obama's administration. In spite of a significant drawdown of U.S. ground forces in Syria at the direction of President Donald Trump in 2019, the United States retains a residual presence of about 600 military personnel in Syria, and continues to conduct airstrikes against Iranian-supported militias as of 2021. The United States House of Representatives voted to repeal the 2001 AUMF in 2021.[42]
Iraq War[43]	Ba'athist Iraq	H.J. Res. 114, March 3, 2003	77–23	296–132	George W. Bush	During the Iraq disarmament crisis Bush successfully requested an authorization of military force against Iraq citing alleged allegations of violations of United Nations Security Council resolutions including the ceasefire with Kuwait, allegations of illegal weapons of mass destruction programs, allegations of the sheltering of al-Qaeda members in the country. The Bush administration also claimed that the conflict was sanctioned by the 1991 AUMF against Iraq and by the Iraq Liberation Act of 1998 designating the overthrow of Saddam Hussein's dictatorship as a goal of U.S. foreign policy.[36][44]	Ba'athist Iraqi government abolished April 2003, Saddam Hussein executed. War ended December 15, 2011. Destabilization of Iraq and emergence of ISIL (ISIS) in Iraq region 2014–2017.[45] During the 2019–2021 Persian Gulf crisis, President Donald Trump cited the AUMF in its assassination of Qasem Soleimani.[42] The United States House of Representatives voted to repeal the AUMF in 2021.[42]

In many instances, the United States has engaged in extended military engagements that were authorized by United Nations Security Council Resolutions and funded by appropriations from Congress.^[46]

Military engagement	Opponent(s)	Initial authorization	President	Result
Korean War	China  North Korea  Soviet Union	UNSCR 84, 1950	Harry S. Truman	Korean Armistice Agreement,[47] 1953
Multinational Force in Lebanon	Shia militias, Druze militias,  Syria	UNSCR 425, 1978 UNSCR 426, 1978	Jimmy Carter, Ronald Reagan	U.S. forces withdrew in 1984
Persian Gulf War	Iraq	UNSCR 678, 1990	George H. W. Bush	UNSCR 689, 1991
Bosnian War	Republika Srpska	UNSCR 770, 1992 UNSCR 776, 1992 UNSCR 836, 1993	Bill Clinton	Reflagged as IFOR in 1995, Reflagged as SFOR in 1996, Completed in 2004
Second Liberian Civil War	N/A (Peacekeeping)	UNSCR 1497, 2003	George W. Bush	U.S. forces are withdrawn in 2003 after the UNMIL is established.
Haitian coup d'état		UNSCR 1529, 2004 UNSCR 1542, 2004		2004
First Libyan Civil War 2011 military intervention in Libya Operation Odyssey Dawn Operation Unified Protector	Libya	UNSCR 1973, 2011	Barack Obama	Debellation of the Libyan Arab Jamahiriya, October 31, 2011

On at least 125 occasions, the president has acted without prior express military authorization from Congress.^[48] These include instances in which the United States fought in the Philippine–American War from 1898 to 1903, in Nicaragua in 1927, as well as the NATO bombing campaign of Yugoslavia in 1999, and the 2018 missile strikes on Syria.

The United States' longest war, against the Taliban in Afghanistan, began in 2001 and ended with the withdrawal of American troops on 31 August 2021.^[49]

The Indian Wars comprise at least 28 conflicts and engagements. These localized conflicts, with Native Americans, began with European colonists coming to North America, long before the establishment of the United States. For the purpose of this discussion, the Indian Wars are defined as conflicts with the United States of America. They begin as one front in the American Revolutionary War in 1775 and had concluded by 1918. The United States Army still maintains a campaign streamer for Pine Ridge 1890–1891 despite opposition from certain Native American groups.^[50]

The American Civil War was not an international conflict under the laws of war, because the Confederate States of America (CSA) was not a government that had been granted full diplomatic recognition as a sovereign nation by other sovereign states^[51][52] or by the government of the United States.^[53]

In 1973, following the withdrawal of most American troops from the Vietnam War, a debate emerged about the extent of presidential power in deploying troops without a declaration of war. A compromise in the debate was reached with the War Powers Resolution. This act clearly defined how many soldiers could be deployed by the president of the United States and for how long. It also required formal reports by the president to Congress regarding the status of such deployments, and limited the total amount of time that American forces could be deployed without a formal declaration of war.

Although the constitutionality of the act has never been tested, it is usually followed, most notably during the Grenada Conflict, the Panamanian Conflict, the Somalia Conflict, the Persian Gulf War, and the Iraq War^{[clarification needed]}. The only exception was President Clinton's use of U.S. troops in the 78-day NATO air campaign against Yugoslavia during the Kosovo War.^{[citation needed]} In all other cases, the president asserted the constitutional authority to commit troops without the necessity of congressional approval, but in each case the president received congressional authorization that satisfied the provisions of the War Powers Act.

On March 21, 2011, a number of lawmakers expressed concern that the decision of President Barack Obama to order the U.S. military to join in attacks of Libyan air defenses and government forces exceeded his constitutional authority because the decision to authorize the attack was made without congressional permission.^[54] Obama explained his rationale in a two-page letter, stating that as commander in chief, he had constitutional authority to authorize the strikes, which would be limited in scope and duration, and necessary to prevent a humanitarian disaster in Libya.

• Cold War
• Declaration of war by Canada
• Declaration of war by the United Kingdom
• Just War Theory
• Police action
• Timeline of United States military operations
• War on Terror
• War on Drugs

• Grotius, Hugo (2004). On The Law Of War And Peace. Kessinger Publishing. ISBN 978-1-4191-3875-1.
• Edwin Meese; Matthew Spalding; David F. Forte (2005). The Heritage guide to the Constitution. Regnery Publishing. ISBN 978-1-59698-001-3.
• Kenneth A. Schultz, Tying Hands and Washing Hands: The U.S. Congress and Multilateral Humanitarian Intervention, Ch. 4, pp 105–142, in Daniel Drezner, Ed. Locating the Proper Authorities: The Interaction of Domestic and International Institutions, University of Michigan Press, 2003.

• The House of Rep, Republican Study Committee of War and Military Authorized Conflicts. 2003.
• Declarations of war and votes
• Text of Declaration of War on Japan
• Text of Declaration of War on Germany
• Text of Declaration of War on Bulgaria
• Authorization for Use of Military Force — signed September 18, 2001
• House Joint Resolution Authorizing Use of Force Against Iraq — signed October 16, 2002
• Instances of Use of United States Forces Abroad, 1798–1993
• A partial list of U.S. military interventions from 1890 to 2006
• U.S.-Africa Chronology