After brilliant studies, he entered in 1914 the École Normale Supérieure (ENS) after having been the student of Alain, a philosopher who was active in the Radical Party and who would write a deeply anti-militarist book after World War I.

When the war ended in 1918, he finished his studies at the École Normale and passed his agrégation of philosophy, and oriented himself towards sociology under the direction of Célestin Bouglé,[1] a friend of Alain and also member of the Radical Party.

Déat and other Neosocialists were expelled from the SFIO at the 5 November 1933 Congress, for their revisionist views and disagreements with Léon Blum's policies toward Prime Minister Édouard Herriot, leader of the second Cartel des Gauches (Left-Wing Coalition).

The Neosocialists wanted to "reinforce the state against the economic crisis", open themselves to the middle classes and participate in non-Socialist governments.

He returned to the Chamber of Deputies in 1936 as a delegate from Angoulême, and at first supported the Popular Front led by Blum before denouncing "Communist infiltration" of it.

[3][4] There, he argued that France should avoid war with Germany if the latter seized Poland – the publication caused a widespread controversy, and propelled Déat to national fame.

He attempted to create a single party to fully realize the aims of the "Révolution nationale", the official, reactionary ideology of Vichy.

Déat also founded, along with fellow Collaborationists Jacques Doriot and Marcel Bucard, the Légion des Volontaires Français (LVF), a French unit of the Wehrmacht (later affiliated with the Waffen-SS).

While reviewing troops from the LVF with former Prime Minister Pierre Laval in Versailles on 27 August 1941, Déat was wounded in an assassination attempt—carried out by Paul Collette.

He was later taken in and hidden by a Roman Catholic religious order in the convent of San Vito, near Turin, where he wrote his memoirs and lived undiscovered until his death in 1955.