The military air forces formed in Great Britian during World War I merged in 1918 into the Royal Air Force (RAF). The Women’s Auxiliary Air Force (WAAF) formed in June 1939 to support operations of the RAF. Great Britain declared war on Germany in September 1939. After Dunkirk, when trapped Allied troops were evacuated over several days in May and June 1940 to avoid surrendering to German forces, Hitler’s armies won the Low Counties and part of France. Hitler then prepared to invade Britain, beginning with assaults from the air by the German air force, the Luftwaffe.

In July 1940, German bombers began attacking British supply centers, airfields, ports, factories, and radar stations. Between July 1940 and September 1940, the defense waged by the British RAF, known as the Battle of Britain, managed to quell the attacks at the cost of high casualties among pilots. Pilots were recruited from across the British Commonwealth, and volunteers were accepted from other countries. Though it was against United States law for citizens to join foreign militaries, many Americans volunteered for the Royal Air Force, often pretending to be Canadian. However, in December 1941, following the Japanese attack on Pearl Harbor, the United States entered the war.