I te 1940, i awhina a Marcantonio ki te whakatuu i te American Peace Mobilization (APM), he roopu ko te kaupapa kia kaua te US e uru ki te Pakanga Tuarua o te Ao (he rite tonu te kaupapa ki te Komihana Tuatahi o Amerika. I mua i te Molotov-Ribbentrop Pact (he kawenata Nazi-Soviet kore whakaeke) i hainatia i Moscow 23 Akuhata 1939, ko te whakahaere o mua o te APM, te Comintern-i arahina e te American League for Peace and Democracy, he anti-Nazi. Ko Marcantonio te tumuaki tuarua o te APM. I puta ia i roto i tetahi purongo i te tau 1940 e whakahe ana i te 'whawhai imperialist', te raina i tangohia e Joseph Stalin me ana kaitautoko i te Soviet Union tae noa ki te whakaekenga o te Soviet Union e te Tiamana Nazi. I mau tonu te Tiriti tae noa ki te wa i pakaru ai nga Tiamana ma te whakaeke i te Soviet Union i te 22 o Pipiri 1941. I te tau 1942 i mahi a Marcantonio ki te whakawhānui i te piripono o te ope taua o U.S. ki te taha tuarua i Uropi
ki te whakawhānui i te Nazi. I whakahau te USSR i nga Communists puta noa i te ao ki te whakatairanga i te whakaaro hei awhina i a ia ki te hinga i te Nazis. Ko Marcantonio hoki te Perehitini Tuarua o te International Workers Order, he hapori painga teina kaore i te hono ki te Rōpū Communist.