American imperialism emerged as the U.S. sought global influence through economic and military means. This era saw the Spanish-American War, acquisition of territories, and policies like the Open Door Policy and Roosevelt Corollary. World War I marked a turning point for the U.S. Initially neutral, factors like unrestricted submarine warfare and the Zimmermann Telegram led to U.S. involvement. The war's impact reshaped American society and the global balance of power.