He established contact with clandestine anti-fascist networks at age 16, getting arrested in 1938 and being sentenced to 7 years in prison for "subversive activities" including the distribution of left-wing pamphlets and posters.

[1][2] Upon the fall of Fascism in July 1943, Fanciullacci was released from prison, joining the Partisans against Republic of Salò troops in the civil war 2 months later, choosing "Massimo" as his nom de guerre.

[1][3] After the 22 March 1944, execution of the Martyrs of Campo di Marte, the self-proclaimed "Philosopher of Fascism" and key figure of Benito Mussolini's Fascist government Giovanni Gentile received a death threat in the form of a letter, now believed to have been sent by Fanciullacci or his GAP associates.

[8][1] On 9 July 1944, Bruno Fanciullacci and Elio Chianesi led a GAP squadron in a raid against the Santa Verdiana monastery, where some anti-fascists were being held prisoner.

[1][3] A few days later, Fanciullacci and Chianesi were captured by Mario Carità's Fascist forces in a trap set at the Piazza Santa Croce, having hidden themselves in an ambulance car.