He became active against Benito Mussolini's Fascist regime and joined Carlo and Nello Rosselli's Giustizia e Libertà) ("Justice and Liberty"), the most important Italian non-Marxist anti-fascist movement.

[4] In 1926, together with Carlo Rosselli and future President of Italy Sandro Pertini he was involved in planning and assisting the escape to France of reformist Socialist leader Filippo Turati.

[6] Parri remained in contact with Giustizia e Libertà, and in 1942 founded the Action Party, an anti-fascist liberal socialist movement that sought to pair social justice and respect for civil liberties.

Living underground in Nazi-occupied Northern Italy, he became a member of the National Liberation Committee[7] and deputy commander of the main group of partisan forces, the Corpo Volontari della Libertà.

He was held prisoner until March when he was released as part of Operation Sunrise – a series of secret negotiations between Allen Dulles, head of the U.S. Office of Strategic Services (OSS) and representatives of the German Wehrmacht command in Northern Italy.

By the time the war ended the Giustizia e Libertà Brigades, the military arm of the Action Party, were the second largest partisan units, accounting for about 20% of all fighters of the Italian resistance movement.

[11] In spite of the wartime strength of Giustizia e libertà the Action Party quickly faded from the Italian political scene, winning 1.46% in the 1946 Constituent Assembly election.

In 1953 Parri, who was opposed to recent changes to the election law, left the PRI to establish the short-lived Popular Unity (Unità Popolare; UP) with former Action Party member Piero Calamandrei, with the goal of preventing the centrist coalition from winning a majority bonus of seats.

My job is not only to prevent the right and left wings from exercising undue influence on the Government, but I have to think too of the enormous masses of peasants sweating in the fields under the sun, blacksmiths beating their anvils in villages, workers, men and women everywhere who have no taste for politics and are outside parties. ...