The United States has officially declared war 11 times during five separate military conflicts. According to the Constitution (Article I, Section 8), Congress has the exclusive power to declare war. The last time America declared war was during World War II. The Korean War, the War in Vietnam, and the extended campaigns in Afghanistan and Iraq were never stamped with congressional declarations of war. 

1. War of 1812

Known as the “second war of independence,” the War of 1812 was America’s first military test as a sovereign nation. President James Madison angered at Great Britain’s refusal to respect America’s neutrality in the ongoing conflict between Great Britain and France, asked Congress to declare war on its former colonial overlord. 

The vote in Congress wasn’t unanimous, with Federalists objecting to the aggressiveness of newly-elected “war hawk” legislators. But in the end, the House voted 79-49 and Senate 19-13 to wage war on what remained the greatest military power on Earth. 

Madison signed the declaration on June 18, 1812. “[W]ar . . . is hereby declared to exist between Great Britain and her dependencies and the United States of America and their Territories,” read the decree. “[A]nd ...the President of the United States is hereby authorized to use the whole land and naval force of the United States to carry the same into effect.”

2. Mexican-American War

Battle of Monterey, 1846, The Mexican American War
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The Battle of Monterey, illustrated here, occurred on July 7, 1846, during the Mexican-American War.

The 1846 War with Mexico started as a land dispute. In 1836, Texas won independence from Mexico to become the Republic of Texas, but Mexico never relinquished its claim on that land. So when the United States annexed Texas in 1845, tensions escalated between the northern and southern neighbors. When President James Polk sent U.S. troops to patrol the Rio Grande border, the Mexican Army attacked, giving Polk the justification he needed to ask Congress to declare war. 

Congress was even more divided about going to war with Mexico than the War of 1812. Northern Whigs saw it as an unjustified land-grab by Southern Democrats who were looking to add more slave-owning territory to the United States. 

Ultimately, the Whigs relented, fearing that they’d suffer the same political fate as the Federalists, whose opposition to the War of 1812 led to their doom. The Senate passed the declaration on May 12, 1846, which began, “Whereas, by the act of the Republic of Mexico, a state of war exists between that Government and the United States.”

3. Spanish-American War

The short-lived war between the United States and Spain began as a Cuban war for independence. American newspapers closely followed the plight of Cuban revolutionaries as they fought with Spain from 1895 to 1898, publishing sensational tales that were criticized as “yellow journalism.” 

America’s involvement in the nearby conflict was sealed by the mysterious sinking of the U.S. warship Maine in Havana harbor on February 15, 1898. 

Congress passed a resolution in April recognizing Cuban independence and ordering Spain to back down, but Spain refused. So President William McKinley responded with a U.S. naval blockade of Cuba and called for 125,000 volunteers to enforce it. Spain immediately declared war and the U.S. Congress followed suit on April 25, 1898. 

The declaration was passed unanimously by a voice vote in both chambers. The war ended on December 10 with the Treaty of Paris, in which Spain not only granted Cuba its independence but ceded the territories of Guam and Puerto Rico to the United States. Spain also agreed to sell the Philippines to the United States for $20 million. 

4. World War I - Germany

President Woodrow Wilson was not eager to enter World War I. Even after the 1915 sinking of the British passenger liners the Lusitania and the Arabic by German submarines, in which 131 U.S. citizens were killed, Wilson held back. Instead of declaring war, he made the Germans promise to halt attacks on Allied civilian ships.

By 1917, however, the Germans had recalculated and decided to risk U.S. involvement in World War I by reinstating their attacks on civilian vessels in the North Atlantic, believing they could win the war before the Americans could enter it. 

On April 2, 1917, Wilson asked Congress to declare war on Germany, citing its resumed submarine attacks and its attempts to recruit Mexico as an enemy of the United States. The declaration passed by large margins in both the House (373-50) and Senate (82-6). U.S. forces suffered more than 320,000 in World War I, including more than 116,000 deaths. 

5. World War I - Austria-Hungary

Archduke Franz Ferdinand and his wife Sophie shortly before their assassination, and a page from Le Petit Journal, illustrating the assassination. (Credit: Henry Guttmann/Getty Images & Popperfoto/Getty Images)
Archduke Franz Ferdinand and his wife Sophie shortly before their assassination, and a page from Le Petit Journal, illustrating the assassination. (Credit: Henry Guttmann/Getty Images & Popperfoto/Getty Images)

On July 28, 1914, Austria-Hungary declared war against Serbia in retaliation for the assassination of Archduke Franz Ferdinand of Austria and his wife by Serbian nationalists. Germany immediately allied itself with its Austria-Hungary, triggering a political domino effect that brought all of the great European powers to war within a matter of days. 

On December 10, eight months after the United States declared war on Germany, Congress passed a separate declaration of war against Austria-Hungary, citing the empire’s complicity in German submarine attacks on American ships. 

6. World War II - Japan

In the early morning hours of December 7, 1941, Japan launched a devastating surprise attack on the U.S. naval facilities at Pearl Harbor, Hawaii. After less than two hours of aerial bombardment, most of the Pacific fleet was sunk and 3,500 American servicemen had been killed or wounded. 

That very day, President Franklin D. Roosevelt penned his iconic speech to Congress asking for a declaration of war against Japan for the “unprovoked and dastardly attack.” And on December 8, Roosevelt addressed Congress and the nation, calling December 7, 1941 “a date which will live in infamy.” Congress’s response was swift, with a near-unanimous roll call vote in the House (the single dissenter was pacifist Jeannette Rankin of Montana) and a unanimous declaration of war in the Senate. 

7. - 8. World War II - Germany and Italy

A 1939 poster shows a fist holding a stamp with an American star ready to stamp out a Nazi swastika during World War II.
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A 1941 Works Progress Administration poster shows a fist holding a stamp with a U.S. star ready to stamp out a Nazi swastika during World War II.

Just four days after the Japanese attack on Pearl Harbor, with Americans still reeling with grief and anger, the Nazi leader Adolf Hitler of Germany delivered yet another surprise by declaring war on the United States. Fascist Italy, bound by an Axis pact signed in 1940, also declared war on America.

On December 11, 1941, Roosevelt sent a message to Congress asking once again for a declaration of war. “The forces endeavoring to enslave the entire world now are moving toward this hemisphere,” wrote Roosevelt. “Rapid and united effort by all of the peoples of the world who are determined to remain free will insure [sic] a world victory of the forces of justice and of righteousness over the forces of savagery and of barbarism.”

This time there were absolutely no dissenters. In the House, Rankin chose to vote “present” instead of “nay,” making the vote 393 to 0. 

9.-11. World War II - Bulgaria, Hungary and Romania

On June 3, 1942, Roosevelt signed three final declarations war on the remaining Axis powers. Bulgaria, Hungary and Romania each had their own reasons for allying with Germany in 1940. Bulgaria had territorial disputes with Yugoslavia and Greece and thought Germany could provide some muscle. Hungary was afraid of being swallowed up by the Soviet Union. And Romania was ruled by fascists and antisemites who sided with the Nazis.

In his letter to Congress, Roosevelt wrote, “I realize that the three Governments took this action [declaring war on the United States] not upon their own initiative or in response to the wishes of their own peoples but as the instruments of Hitler.” 

These three declarations of war during World War II were the last ever passed by the United States Congress. All ensuing wars—the Korean War, the War in Vietnam, the wars in Afghanistan and Iraq—were either initiated by a congressional “authorization of use of military force” (AUMF), or in the case of Korea, not authorized by Congress at all.