Rachman was born in Lwów, Poland, in 1919, the son of a Jewish dentist. After the German invasion of Poland in 1939, Rachman may have joined the Polish resistance. He was first interned by the Germans and, after escaping across the Soviet border, was reinterned in a Soviet labour camp in Siberia and cruelly treated. After the Germans invaded the Soviet Union in 1941, Rachman and other Polish prisoners joined the II Polish Corps and fought with the Allies in the Middle East and Italy. After the war he stayed with his unit, as an occupation force in Italy until 1946 when it transferred to Britain. Rachman was demobilised in 1948 and became a British resident.