Sidos was convicted in 1946 of joining the fascist Mouvement Franciste at 16 years old in the midst of WWII and Nazi occupation of France. After spending two years of internment in Natzweiler-Struthof, he founded in 1949 Jeune Nation, the most prominent French neo-fascist movement of the 1950s. Famous for its insurrectional violence during the Algerian war, the organization was dissolved by official decree in 1958.