http://www.historyguy.com/worldwartwo/when_was_world_war_2.htm
Japanese soldiers at Marco Polo Bridge.
When Was World War Two?
When was World War Two? This seems like an easy question, but it can be an elusive answer. There are several answers to that question, as many historians debate when World War Two began. The end of World War Two is fairly simple to answer, as the Japanese surrendered on September 2, 1945 in Tokyo Bay.So, when did World War Two begin? Depends on which part of the war you look at.
There are several competing dates for the starting point. If we look at World War Two as a truly global war (which of course it was), and not looking at it from the European or Western point of view, we can pin the answer down to only two dates:
September 18, 1931–The Mukden Incident (also known as the Manchurian Incident) was a pretext for the Japanese invasion and occupation of the region of China known as Manchuria.
July 7, 1937–the Marco Polo Bridge Incident. This is when Japan (one of the Axis powers of World War Two) began its massive invasion of China.
Many historians prefer the 1937 date over the 1931 incident as the Marco Polo Bridge incident led to a major war between China (which became one of the Allies of World War Two), and Japan and Germany had already, in November of 1936, signed an Anti-Comintern Pact that made them allies against the democracies and against the Soviet Union.
The start of the European part of World War Two is a bit clearer, as most historians put the start date with the German Invasion of Poland on September 1, 1939.