Thomas was born in a Jewish family who moved to Paris when he was 6. His father's outspoken criticism of the fascist government and his affiliation with the Hungarian Communist Party led to the Family's expulsion from Hungary in 1930.
Thomas's father often argued against fascism, and he was greatly disturbed when Hitler became chancellor of Germany in 1933... He was upset to learn that Hungary, his mother country, had instituted anti-Jewish laws.
Thomas participated in sabotage actions against the Germans. His group launched numerous grenade attacks, and set fire to a German library. On July 28, 1943, his unit blew up a convoy of German officers and soldiers, killing 600.