The dissertation is a historical biography of Smedley Darlington Butler (1881-1940), a decorated soldier and critic of war profiteering during the 1930s. A two-time Congressional Medal of Honor winner and son of a powerful congressman, Butler was one of the most prominent military figures of his era. He witnessed firsthand the American expansionism of the late nineteenth and early twentieth centuries, participating in all of the major conflicts and most of the minor ones. Following his retirement in 1931, Butler became an outspoken critic of American intervention, arguing in speeches and writings against war profiteering and the injustices of expansionism. His critiques represented a wide swath of public opinion at the time - the majority of Americans supported anti-interventionist policies through 1939. Yet unlike other members of the movement, Butler based his theories not on abstract principles, but on experiences culled from decades of soldiering: the terrors and wasted resources of the battlefield, the use of the American military to bolster corrupt foreign governments, and the influence of powerful, domestic moneyed interests. Butler's story is reminiscent of a comment Mark Twain once made about America: "This nation is like all the others that have been spewed upon the earth - ready to shout for any cause that will tickle its vanity or fill its pocket." That was Smedley Butler in his early years - a soldier shouting for the "cause." Later, he would decide to expose those whose pockets he had helped to fill. This ideological shift - from imperialism to isolationism - rippled through homes across the country in the interwar period as it did in Butler's mind. In this dissertation, I will examine this ideological movement through the study of one of its most prominent leaders.