God, that's a terrible question. What's their rise to power, fwiw? Are we talking about Spanish-American (generally marked as their entrance to the global stage if I want to be superficial) or WWII? If you want an interesting take, skip the wars.
I'd argue US Industrialization was the biggest factor that put the US on the world stage. So the interesting question is which of those categories is the most important. Laissez Faire policies could be one (capitalism, maybe?). Population and immigration (capitalism? democracy?).
Imperialism as an overarching theme is interesting with Manifest Destiny and the evolution of the Monroe Doctrine. However, US Imperialism was never quite the same as others. There was always a significant opposition by those who felt it was against US ideals. Certainly, some, like the annexation of the Philippines, had to be justified by free trade (even though many just wanted to be imperialists). In China, the US fought for an Open Door policy (US access to all ports) rather than a slice of China to control.