1910s: By July 1918, the United States has sent one million troops to Europe to fight in World War I (1914-1918), a force augmented by an average of 200,000 men per month until the armistice is signed on November 11, 1918. After the war ends, certain works of literature and people in wider society ask whether the war was necessary.

1960s: In 1961, in response to a perceived Communist threat, the United States deploys 4,000 troops in South Vietnam. By July 1965, some 75,000 U.S. troops are in Vietnam. The figure continues to climb to more than 510,000 early in 1968. Opposition to U.S. involvement in the war begins in 1964 on college campuses. Protests against the draft begin in 1965, when the student-run National Coordinating Committee to End the War in Vietnam stages the first public burning of a draft card in the United States.

Today: Between 2003 and 2005, many global protests are held...