Ranking Communist Party (CPUSA) figure, American Trotskyist movement founder, and longtime Socialist Workers Party leader, James Cannon was born in Rosedale, Kansas, on February 11, 1890. Poverty radicalized his immigrant father, John, a laborer in a foundry. John introduced the young James to Irish nationalism, the Knights of Labor, populism, and finally the Socialist Party. James displayed a deep and lifelong anger at capitalism and the injustices it spawned. He left high school to organize for the Industrial Workers of the World. Soon he joined the Socialist Party and rose rapidly among its left wing. Cannon had taken a path that would make him an acquaintance of nearly all the era's celebrated radicals.

After Earl Browder's imprisonment during World War I, Cannon edited , the organ of the Socialist Party in Kansas. In 1919 the party expelled its left wing, and Cannon became prominent in the nascent Communist underground. He held important St. Louis and Cleveland posts, then transferred to New York City in 1921. There he helped head the Workers Party, a legal organization, after the "Red Scare" subsided and V. I. Lenin shifted toward nonrevolutionary tactics. Between 1922 and 1928 Cannon served on the Communist International's presidium and headed International Labor Defense, a leftist legal support group. He also backed William Z. Foster's faction of what had become the CPUSA.

Cannon's life took a dramatic turn in 1928 while he was in the Soviet Union. In Moscow, Leon Trotsky offered compelling criticism of Joseph Stalin and the Soviet Union's direction. Cannon returned to the United States carrying Trotsky's writings. That October, Cannon and 100 followers were driven from the cpusa. They called themselves the Communist League of America (Left Opposition), began publishing a newspaper, , and later a magazine, . From that time on, Cannon would consider Trotsky the custodian of authentic Bolshevik values. The League helped lead a sensational Minneapolis Teamster strike, fused with A. J. Muste's followers, and in 1936 entered the Socialist Party. The Trotskyists were expelled the following year, and Cannon founded the Socialist Workers Party in 1938. By 1939 the movement had 1,000 American members and a following in labor and intellectual circles.

Success brought schism during the early months of World War II. Socialist Workers Party figures Max Shachtman and James Burnham argued that the Stalinists constituted a new bureaucratic class in the Soviet Union. Cannon, like Trotsky, considered the Soviet Union's nationalized economy worth defending, despite Stalin's dictatorship. In early 1940 Shachtman's faction became the Workers Party. In 1941 federal authorities used the Smith Act to imprison Cannon and seventeen other Socialist Workers Party figures and Minneapolis Teamsters. Supposedly their organizations had advocated the violent overthrow of the U.S. government; actually they opposed American foreign policy. Cannon served thirteen months at Sandstone Penitentiary. During the McCarthy era, a public that drew few distinctions among radicals persecuted the Socialist Workers Party and Communist Party alike.

Cannon retired in 1953 but lived to see the "New Left" of the 1960s and 1970s. He died on August 21, 1974, believing that Vietnam War resisters, African Americans, Hispanics, women, youth, and gays were taking a permanent step toward socialism in the United States.