What did Spain lose at the end of the Spanish-American War?
Cuba, Guam, Philippines and Puerto Rico.
America's prosperity had always been based on expansion and growth. As the arable parts of the Southwest got thoroughly settled, American business interests looked increasingly at Cuba and the Philippines as areas to expand into. Both were Spanish possessions.
War with Spain was declared in 1898 under very flimsy, suspicious circumstances. It ended less than four months later with the US in possession of Cuba, the Philippines, Puerto Rico and Guam.