Jerzy Lerski first saw action in World War II during the 1939 Polish September Campaign. He fought in the Battle of Brześć Litewski in the rank of officer cadet ("podchorąży", ensign). Taken prisoner by the Soviet Army during the Soviet invasion, he managed to escape from a train transport. He joined the anti-Soviet resistance but, with the NKVD (the Soviet Secret Police) actively looking for him, he escaped via Hungary to France, joining the Polish Armed Forces in the West. After the fall of France on 25 June 1940, Lerski went to Great Britain. In 1941, after taking a commando course, he became a "cichociemny" (a member of a secret unit of the Polish Army in exile). Parachuting into occupied Poland in February 1943, he served as one of the couriers for the Polish Government in Exile – individuals who risked their lives, moving between the Polish Government in Exile, in London, England, and the Polish Secret State in occupied Poland. After collecting information from the Secret State, he returned to London.