Historians generally refer to the "long 19th century" as encompassing the time from the French Revolution to WWI (although there is some room to discuss at what exact year you want to place the endpoints). Post-WWI periodization is hard to discuss because it's not necessarily clear until long after what events make more of a difference.
WWII isn't really a single war as much as it is a collection of several wars, be they wars of territorial expansion (e.g., Germany, USSR), ideological civil war (e.g., China), non-ideological civil war (e.g., France), or random local conflicts (e.g., Yugoslav coup) which are all occurring in the context of one another. In many places, it's often a three-way war, with all parties equally at odds with one another, instead of the simple Axis v. Allies it's usually presented as. Start including the full context of these conflicts into the timeframe of WWII, and you push its start date back arguably to 1931, only 8 years after the last cleanup war of WWI (the Russian Civil War).
Viewed from an international relations perspective, combining WWI and WWII into a single period of flux makes sense. WWI makes the death of great power influence, but the attempted resolution afterwards (League of Nations) was impotent and unable to have any major effect on the international crises that arose, unlike the great power conferences preceding WWI. Instead, the end of WWII ushers in the era of superpower relations, where essentially the USA and USSR drive international relations for the rest of the world.