In September 1939 World War II began with the invasion of Poland. Sperrle and his air fleet served exclusively on the Western Front. He played a crucial role in the Battle of France and Battle of Britain in 1940. In 1941 Sperrle directed operations during The Blitz over Britain. From mid-1941 his air fleet became the sole command in the west. Through 1941 and 1942 he defended German-occupied Europe against the Royal Air Force, as well as the United States Army Air Forces from 1943. Sperrle's command was depleted in the battles of attrition forced on him by the Combined Bomber Offensive.