In Tours (France), where he organized immigrant laborers, Epstein met and married Paula Grynfeld, a Jewish pharmacy student from the city of Łódź in Poland.
When the French police were informed by the Polish consulate about his participation in the Communist Party, Epstein was arrested and forced to leave Tours.
In 1936, Epstein joined the Republican side in the Spanish Civil War and took part in the defence of Irun where he was severely wounded.
On returning to France at the end of 1938, he was imprisoned at Gurs,[2] a detention camp for political refugees and members of the International Brigades.
In 1941, he began working with Francs-Tireurs et Partisans (FTP), a communist resistance group; by February 1943, he was an operation commander in the Paris region.
Up till this point, the FTP had been operating in three-person cells: one person attacked and two provided covering fire for their escape.
The FTP knew that a formation of Wehrmacht soldiers would take part in a parade in a street leading to place de l'Étoile.