Britain entered the war after Germany invaded Poland on 1 September 1939. Pownall held a series of command and senior staff positions throughout the war. He was appointed Chief of General Staff for the British Expeditionary Force (BEF) in France and Belgium until the fall of France in May 1940. He then assumed the position of inspector general for the recently created Home Guard and was Commander of British Troops in Northern Ireland, before being appointed the Vice Chief of the Imperial General Staff in the War Office in 1941.