[DirtyDick]

There is another view concerning when WWII became a World War.

One contention is that until the USA and Japan entered the War in late 1941 it was only yet another a European struggle, broken down, broadly, into an Anglo-Franco vs Germany campaign and an Anglo-Russian vs Germany and Italy; those in the Far East being an Asian struggle (equivalent of the Sino-Japanese War of 1894) rather than the first shots of WW2.

The Seven Years' War of 1756-63 has also been ascribed the status of a World War by some, since it was fought by much of Western Europe across the globe (a product of imperial expansion and the protection of trade routes).

Semantics and terminology really, so no clear cut off point; although 1939 is generally chosen for purposes of clarity since it marked the start of hostilities between the major European powers and was not ended (at least in the case of UK & Empire) until 1945.