September 1st, 1939. For Poland, Great Britain, Canada, and France this was the start of the War. For those who resided in Czechoslovakia, WW 2 began in March of that year (1939) when Germany attacked the country. For the Chinese, the war began in 1931 with the Japanese occupation of Manchuria, although those in the west believed it to be a regional conflict at the time. Russia counts the start of World War 2 in June of 1941, while in the United States, active involvement in WW 2 begins with the Japanese attack on Pearl Harbor on December 7th, 1941. The first official declaration of war involving western countries was on September 3rd, 1939 when France and Great Britain declared war on Germany due to treaty obligations with Poland.
Lone wolf
Different history source - difference dates: