That's a fair take. Before this year, most of us might have said 9/11 was the defining moment of the start of this century.

I'm not going to try to predict what history will look like 80 years from now, or whether 9/11 or covid will be considered more significant by then, but this does raise an interesting question of how people in 1913 perceived the nascent 20th century.

Obviously things happened over those 13 years, but whatever they were, history ultimately decided that they were inconsequential enough for the "20th-century events" Wikipedia article to not even mention anything before WWI: https://en.wikipedia.org/wiki/20th-century_events

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.

To me it's quite obviously a preview of the ecological collapses that climate change will bring over the rest of the century. This is a small taste of things to come and the scale of the solutions and sacrifices required to adapt to them. The amount of government intervention, which was met with a lot of skepticism in the West, is going to remain a prominent feature.

WWI marks the end of the Concert of Europe system that prevailed over the previous century, and the cost of WWI and its subsequent aftershocks led to the collapse of European global empires and the rise of the American/Soviet century. I'd say that's a pretty good demarcation line.