The first national convention of the American Legion was held from November 10-12, 1919, in Minneapolis, Minnesota, at which time the attendees adopted a permanent constitution and elected officers to head the organization. The original purpose of the Legion was to "preserve the memories and incidents of our association in the great war". Prior to World War I, few rural, working-class, or even middle-class Americans traveled to Europe. For a majority of urban Americans, their understanding of Europe had been acquired through the European immigrants they knew. Thus the 2 million Americans who had served in the American Expeditionary Forces had very different experiences than their families, friends and neighbors. The American Legion allowed these young men and women who had served "Over There" to re-integrate into their hometowns and to still remain in contact with others who had been abroad. The Legion served as a support group, a social club and a type of extended family for former service men and women.
Some Legion groups engaged in strikebreaking activities during this time and into the 1930s. In 1919, a new American Legion group in Washington was involved in the Centralia Massacre (Washington).
The Industrial Workers of the World had attempted to organize longshoremen, sailors and fishermen in the 1920s through their Maritime Workers Union. The largest strike, in San Pedro, California in 1923, bottled up shipping in that harbor, but was crushed by a combination of injunctions, mass arrests and vigilantism by the American Legion.
In 1930, the American Legion Memorial Bridge in Traverse City, Michigan, was completed. In that year, the Traverse City city commission decided to purchase dedication plaques for $100 at the request of the American Legion.
By 1931, membership of the American Legion had reached 1 million.
The Sons of the American Legion formed at the American Legion's 14th National Convention in Portland, Oregon, on September 12-15, 1932. Membership is limited to the male descendants of members of the American Legion, or deceased individuals who served in the armed forces of the United States during times specified by the American Legion. In 2007, The Sons of the American Legion celebrated 75 years of service to God and Country. The organization has more than 300,000 members.
According to congressional testimony in the 1930s, several of the American Legion's leaders, including its original bankroller Irénée du Pont, plotted a fascist coup against the Government of Franklin D. Roosevelt called the Business Plot. According to testimony, the plot was averted because Major General Smedley Butler warned Roosevelt of the plan.
The American Legion's first National High School Oratorical Contest was held in 1938.