The American Legion, commonly known as the Legion, is an organization of U.S. veterans headquartered in Indianapolis, Indiana. It comprises state, U.S. territory, and overseas departments, in turn made up of local posts. It was established in March 1919 in Paris, France, by officers and men of the American Expeditionary Forces (AEF). It was subsequently chartered by the 66th U.S. Congress on September 16, 1919.

The Legion played the leading role in drafting and passing the Servicemen's Readjustment Act of 1944, commonly known as the "G.I. Bill". In addition to organizing commemorative events, members assist at Department of Veterans Affairs (VA) hospitals and clinics. It is active in issue-oriented U.S. politics. Its primary political activity is lobbying on behalf of veterans and service members, including for benefits such as pensions and the Veterans Health Administration.

The American Legion was established in Paris, France, on March 15 to 17, 1919, by a thousand commissioned officers and enlisted men, delegates from all the units of the American Expeditionary Forces to an organization caucus meeting, which adopted a tentative constitution and selected the name "American Legion".

The aftermath of two American wars in the second half of the 19th century had seen the formation of several ex-soldiers' organizations. Former Union Army soldiers of the American Civil War of 1861–65 established a fraternal organization, the Grand Army of the Republic (GAR), while their Southern counterparts formed the United Confederate Veterans (UCV). Both organizations emerged as powerful political entities, with the GAR serving as a mainstay of the Republican Party, which controlled the presidency from the Civil War through William Howard Taft's administration except during Grover Cleveland's two terms in office. In Southern politics the UCV maintained an even more dominant position as a bulwark of the Democratic Party, which dominated there. The conclusion of the brief Spanish–American conflict of 1898 ushered in another soldiers' organization, the American Veterans of Foreign Service, known today as the Veterans of Foreign Wars of the United States (VFW).

With the termination of hostilities in World War I in November 1918, some American officers who had participated in the conflict began to think about creating a similar organization for the two million men who had been on European duty. The need for an organization for former members of the AEF was pressing and immediate. With the war at an end, hundreds of thousands of impatient draftees found themselves trapped in France and pining for home, certain only that untold weeks or months lay ahead of them before their return would be logistically possible. Morale plummeted. Cautionary voices were raised about an apparent correlation between disaffected and discharged troops and the Bolshevik uprisings in Russia, Finland, Germany, and Hungary.

This situation was a particular matter of concern to Lieutenant Colonel Theodore Roosevelt Jr., President Theodore Roosevelt's eldest son. One day in January 1919, he had a discussion at General Headquarters with the mobilized National Guard officer George A. White, a former newspaper editor at the Portland Oregonian. After long discussion, he suggested the immediate establishment of a new servicemen's organization including all AEF members as well as those soldiers who remained stateside as members of the Army, Navy, and Marine Corps during the war without having been shipped abroad. Roosevelt and White advocated ceaselessly for this proposal until they found sufficient support at headquarters to move forward with the plan. General John J. Pershing issued orders to a group of 20 non-career officers to report to the YMCA in Paris on February 15, 1919. Roosevelt had personally selected these men. They were joined by a number of regular Army officers Pershing selected.

Twenty National Guard and Reserve officers serving in the A. E. F., representing the S. O. S., ten infantry divisions, and several other organizations, were ordered to report in Paris. ... Included in this number were Lieutenant Colonel Theodore Roosevelt, Jr., of the First Division, Lieutenant Colonel Franklin D'Olier of the S.O.S., and Lieutenant Colonel Eric Fisher Wood of the 88th Division. All of these officers have since told me that when they left their divisions they were distinctively permeated with the desire to form a veterans' organization of some comprehensive kind. When they got to Paris they immediately went into conference with the other officers. ... A dinner was spread in the Allied Officers' Club, Rue Faubourg St. Honore, on the night of February 16th... At that dinner the American Legion was born.

The session of reserve and regular officers was instructed to provide a set of laws to curb the problem of declining morale. After three days, the officers presented a series of proposals, including eliminating restrictive regulations, organizing additional athletic and recreational events, and expanding leave time and entertainment programs. At the end of the first day, the officers retired to the Inter-Allied Officers Club, a converted home across the street from the YMCA building. There, Roosevelt told them of his proposal for a new veterans' society. Most of those present were rapidly won to Roosevelt's plan. The officers decided to make all their actions provisional until an elected convention of delegates could convene and did not predetermine a program for the unnamed veterans organization. Instead, they chose to expand their number with a large preliminary meeting, to consist of an equal number of elected delegates to represent both enlisted men and commissioned officers.