Kammala jumla mai zuwa: A cikin 1940, Marcantonio ya taimaka wajen kafa Ƙungiyar Zaman Lafiya ta Amurka (APM), ƙungiyar da ke da burin hana Amurka shiga cikin Yaƙin Duniya na II (don haka kama da manufa na kwamitin farko na Amurka na dama). Kafin a sanya hannu kan Yarjejeniyar Molotov-Ribbentrop (yarjejeniyar rashin cin zarafin Nazi-Soviet) a Moscow a ranar 23 ga Agusta, 1939, kungiyar da ta riga ta fara APM, kungiyar Comintern-directed American League for Peace and Democracy, ta kasance mai adawa da Nazi. Marcantonio ya yi aiki a matsayin mataimakin shugaban APM. Ya bayyana a cikin wani labari a cikin 1940 yana la'antar 'yaƙin mulkin mallaka', layin da Joseph Stalin da magoya bayansa suka ɗauka a Tarayyar Soviet har zuwa mamayewar Tarayyar Soviet ta Nazi Jamus. Yarjejeniyar ta ci gaba har sai Jamus ta karya ta ta mamaye Tarayyar Soviet a ranar 22 ga Yuni, 1941. A cikin 1942 Marcantonio yayi aiki don fadada aikin soja na Amurka zuwa
gaba na biyu a Turai a kan yaduwar Nazi. Tarayyar Soviet ta umarci 'yan kwaminisanci a duk faɗin duniya su tallata ra'ayin don taimaka mata ta kayar da Nazism. Marcantonio ya kuma kasance Mataimakin Shugaban Kungiyar Ma'aikata ta Duniya, wata kungiyar 'yan uwantaka da ba ta da alaƙa da Jam'iyyar Kwaminis.