The main fighting of World War II started in 1939 and ended in 1945. The start date of Sept. 1, 1939 is based on the German army's invasion of Poland. The end date of Sept. 2, 1945 is when Japan signed the documents of surrender.
As World War II included multiple conflicts with different start dates, other dates used for the start of the war include Sept. 19, 1931, when Japan invaded Manchuria on the Asian mainland, and July 7, 1937, when the Second Sino-Japanese War began. The start date of Oct. 3, 1935 indicates the date of the Italian invasion of Abyssinia. Fighting in the European theater of the war ended on May 8, 1945.