Answer:
The global equilibrium, which had allowed the United States to grow and prosper in virtual isolation since 1815 was gone forever as the result of a short but shattering war. In 1898, U.S. domestic support for the independence of Cuba enmeshed the United States in a struggle with Spain over the fate of the island nation. The decision to aid the Cuban resistance was a major departure from the traditional American practice of liberal nationalism, and the results of that decision had far-reaching consequences. The 1898 Treaty of Paris ending the war gave Cuba its independence and also ceded important Spanish possessions to the United States—notably Puerto Rico, the Philippines, and the small island of Guam. The United States was suddenly a colonial power with overseas dependencies.
https://history.state.gov/departmenthistory/short-history/superpower
Explanation:
Size
We were literally able to expand over a massive landmass unchallenged. Sure, Native Americans did provide some resistance, but the technology and numbers of European immigrants were too great to resist. It gave us a ton of room to populate, which is key to being a superpower.
Immigration
Yes, immigration. The US became THE destination of immigrants. It was a new land full of opportunity. It was for anyone who wanted to start a new life based on the idea of general liberty. Sure, there were some caveats and issues, but overall it was incredibly different from what the rest of the world was offering. Immigration brought a lot of different minds and cultures to the US which spurred innovation and advancement.
Resources
Good farmland? Check. Connections to Europe? Check. Resources to mine and trade? Check. Trade relations? Check
Competitors
Canada is sparsely populated and confined to the North. We annexed a lot of Mexico’s land in the Mexican-American War too. The rest of the states in the Americas were a mess and or too small to compete which left the US, who was furthest along in economic development, virtually unchallenged in dominating both North and South America.
War
The US fought a decent amount of wars prior to WWI, but nearly all of them were on their own continent. WWI brought the US the opportunity to engage internationally with other developed nations. While WWI destroyed Europe, the US profited by selling goods to them and swooping in near the end to help break the stalemate. Then comes WWII. Once again, the rest of Europe is destroyed and even the great British Empire has exhausted itself. The new USSR expended millions of soldiers fighting, much more than the US. This leaves the US in a superb position as having no realistic competitor outside the USSR. Come on Cold War!
Money and the Economy
Do you know what the US did during all these years? It advanced. During the 1800s it built a massive industrialized manufacturing base and became and economic powerhouse before war was even a factor. Come WWI and especially WWII, while the world fought, the US sold. WWII made the US a lot of money. Since WWII destroyed Europe, the US was left as the only major power left to produce goods. The US was the developed world for a time after WWII. It spend decades after this cementing its status as the central economy to the rest of the world.
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