After the fall of France and the British retreat to Britain, Maxwell (using the name "Ivan du Maurier", or "Leslie du Maurier", the surname taken from the name of a popular cigarette brand) took part in a protest against the leadership of the Czechoslovak Army, and with 500 other soldiers he was transferred to the Pioneer Corps and later to the North Staffordshire Regiment in 1943. He was then involved in action across Europe, from the Normandy beaches to Berlin, and achieved the rank of sergeant. Maxwell gained a commission in 1945 and was promoted to the rank of captain.