During the Second World War, while studying medicine at university, Bentivegna joined the Italian Communist Party and became an active member of the guerilla groups organized by the Roman resistance [it] following the occupation of Italy by Nazi Germany.

Under the codename "Paolo", he was one of the principle actors of the Via Rasella attack that killed 32 soldiers of the SS Police Regiment Bozen.

According to his own memoirs, Bentivegna became an anti-fascist in 1937 following the introduction of anti-Semitic propaganda and racist legislation in Italy.

[1] Bentivegna co-founded the Trotskyist Group for Marxist Unification (GUM, Gruppo di Unificazione Marxista) in 1939 with Corrado Nourian and Nino Baldini.

[citation needed] His ideology and participation in the demonstrations brought him to the attention of the Italian secret police, who arrested him.