[Looking at Wikipedia, dates given are](https://en.wikipedia.org/wiki/World_War_II) 1 September 1939 – 2 September 1945, which end in the defeat of Japan.

However why is the starting date when Germany invaded Poland and not when the [Japanese invaded China in the Sino-Japanese war](https://en.wikipedia.org/wiki/Second_Sino-Japanese_War) on July 7, 1937, which then ends on 2 September 1945 (same) date as WW2? 

Why is it considered that WW2 started on September 1, 1939 and not July 7, 1937?

**EDIT:** (As per request by @sempaiscuba) Another argument that could be put forward for 1937:

[China was first to sign](http://www.un.org/en/sections/history-united-nations-charter/1945-san-francisco-conference/) UN charter as `first victim of aggression by an Axis power`

So it could be argued that at the time of signing in 1945 consensus was that start was 1937... Otherwise, it should be Poland.