By convention the date generally given by historians is 1 September 1939, when Germany invaded Poland. It ended on 2 September 1945 with the surrender of Japan.
The following other events are occasionally cited as possible starting points: 3 September 1939: Britain and France declared war on Germany following the German invasion of Poland. 7 July 1937: The Japanese invasion of China (the start of the Second Sino-Japanese War). 1931: The Japanese invasion of Manchuria.
Others argue that the two world wars are one conflict separated only by a "ceasefire." From wiki.answers.com
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The following other events are occasionally cited as possible starting points:
3 September 1939: Britain and France declared war on Germany following the German invasion of Poland.
7 July 1937: The Japanese invasion of China (the start of the Second Sino-Japanese War).
1931: The Japanese invasion of Manchuria.
Others argue that the two world wars are one conflict separated only by a "ceasefire."
From wiki.answers.com