Great Britain WW2 By zslamka 1939 Allied Powers Formed Sep 1, 1939 Germany Invades Poland Sep 3, 1939 Great Britain Declares War on Germany 1940 Winston Churchill becomes Prime Minister Jun 10, 1940 North Africa Campaign Jul 10, 1940 Battle of Britain 1941 Soviet Union joins the Allies Jul 9, 1943 Invasion of Sicily Jan 17, 1944 Battle of Monte Cassino 1955 Winston Churchill Resigns After the War