In 1940, Marcantonio helped form the American Peace Mobilization (APM), a group whose aim was to keep the U.S. from participating in World War II (thus similar in aim to the right wing America First Committee). Before the Molotov–Ribbentrop Pact (a Nazi-Soviet non-aggression pact) was signed in Moscow 23 August 1939, the APM's precursor organization, the Comintern-directed American League for Peace and Democracy, had been anti-Nazi. Marcantonio served as the APM's vice-chair. He appeared in a newsreel in 1940 denouncing 'the imperialist war', the line taken by Joseph Stalin and his supporters in the Soviet Union until the invasion of the Soviet Union by Nazi Germany. The Pact lasted until the Germans broke it by invading the Soviet Union on 22 June 1941. In 1942 Marcantonio worked to expand the U.S. military commitment to a second front in Europe against the Nazi expansion. The USSR ordered Communists throughout the world to promote the idea to help it defeat Nazism. Marcantonio was also a Vice President of the International Workers Order, a fraternal benefit society unofficially affiliated with the Communist Party.