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 collap

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.