The term “world war” is a propaganda term. First, the only reason the world was involved was because the world was mostly colonies of the belligerents. In reality it was a European war, and European holdings were involved due to their economics.

In the second one, there were 2 distinct wars where the belligerents were allied for strategic reasons. The US was at war with Japan and Europe was at war.

Since the end, peace has been held with a bunch of strategic alliances, so in any real war, all countries take sides. But with the current 2 notable wars going on, it appears that that alliance structure is breaking down. Alliances are not in line with the economic realities of these countries. The more real things get the less these alliances will hold. This is probably a good thing, as it prevents everything from getting out of hand.