His parents, adherents of "official petit-bourgeois Zionism", left Poland to pursue the Zionist dream in Palestine; the family quickly re-emigrated in 1926 to Belgium.
According to Ernest Mandel, "[c]orrectly establishing the reasons which we had for hope, Leon noted that the workers’ movement in Europe had already reached the lowest point of its ebb.
"[3] At some point, Leon shared an apartment with fellow Trotskyist and resistance leader, the German Martin Monath, to whom he tasked on making work within the Wehrmacht in occupied France.
Leon was also instrumental in making contact with other sections of the Fourth International in wartime Europe – a most difficult and hazardous task; in August 1942, Leon co-organized the first meeting between the Belgian and French Trotskyist parties; he also led the illegal "party work among the proletarian soldiers of the Wehrmacht" and "attended meetings of the underground factory committees" at tremendous risk to himself.
Leon, preparing for that eventuality, played a leading role "in the work of the European Conference of the Fourth International" held in February 1944.
The Nazis wasted no time in deporting Leon to Auschwitz where he was brutalized until he became sick and was then selected by the SS to be sent to the gas chamber.