The most important historical event of the nineteenth century was the defeat of Napoleon, since it set the stage for the Pax Britannica (the British Empire became the global hegemonic power). The decline and fall of the British Empire was the most important historical event of the twentieth century, since this century was not about the communist movements (the rise and fall of the Soviet Union) but about anti-imperialism.
What will be the most important historical event of the twenty-first century? If the USA falls due to a combination of factors, which includes the collapse of its currency, soaring inflation, political scandals, massive civil unrest, and military pressure from its geopolitical adversaries, then that will be the most important historical event. Perhaps history will throw us a surprise and the empire that falls in the twenty-first century will be the one that most intellectuals think is destined to dominate the world in this century and the next: China.
One thing is certain: Only one civilization will make it to the twenty-second century, either the USA or China. The world is too small for both to coexist.