If you asked most people I think they would agree with the 1939-45 timeline. However, as a matter of curiosity I typed the subject line of this topic in Google & this is what it came up with. http://history.sandiego.edu/gen/ww2Timeline/1917-45.html
Quote:

Quote:

I'm quite certain of that.

Sometimes we just have to accept "generally accepted" principles.

The generally accepted principle for the start of WW2 is September 1, 1939, when Germany invaded Poland.

The generally accepted principle for the end of WW2 is August 15, 1945, when Japan surrendered to the United States.

Historical events do not neatly fit our conceptions of "start date" and "end date".  Wars do not start overnight but are the results (in hindsight) of numerous factors.

That's a great question and, I think, worthy of a new topic.

WWI was called "The World War" before WW2.

WW2 is called World War Two (duh, what follows WWI).

WW3 was previously envisioned as a nuclear confrontation between the US/NATO and the Soviet Union/Warsaw Pact because their extensive alliances would have engulfed the entire world.

My preliminary speculation is that a "World War" must involve every continent (except Antarctica if you are going to be picky) and every major government.  It would help if the major governments' very existence depends upon the outcome of the war.

By this definition you could exclude every belligerent action since 1945.