The start of the war is generally held to be 1 September 1939 beginning with the German invasion of Poland; Britain and France declared war on Germany two days later. Other dates for the beginning of war include the Japanese invasion of Manchuria on 13 September 1931; the start of the Second Sino-Japanese War on 7 July 1937; or one of several other events.

Others follow A. J. P. Taylor, who held that there was a simultaneous Sino-Japanese War in East Asia, and a Second European War in Europe and her colonies. The two wars merged in 1941, becoming a single global conflict, at which point the war continued until 1945. This article uses the conventional dating.

The exact date of the war's end is not universally agreed upon. It has been suggested that the war ended at the armistice of 14 August 1945 (V-J Day), rather than the formal surrender of Japan (2 September 1945); in some European histories, it ended on V-E Day (8 May 1945). The Treaty of Peace with Japan was not signed until 1951.