The population of Austria-Hungary immigrated to the United States.

The change of world hegemony does not necessarily lead to war. When the United States succeeded Britain as the world hegemon, although it experienced the Second World War, it was just and different from the previous wars for hegemony. Besides, there is no direct war between the United States and Britain, but it is a peaceful transfer of power. Especially in today's world, with a high degree of economic integration and the existence of nuclear weapons, it is basically impossible for big countries to have a war. Therefore, even if there is a change in hegemony in the future, it is difficult to have a war. As for your question about China surpassing the United States, although China is big, the United States is also big. In fact, China is not much bigger than the United States. Moreover, the United States has more arable land and usable area than China. In terms of population, the birth rate in China is getting lower and lower, and the population aging is very serious. This is a trend, while the population aging in the United States is also very high. However, the United States is an immigrant country, and it can supplement the young population through immigration. Therefore, China may not have the advantages of size and population over the United States. China will not necessarily surpass the United States.