The US became a world power in the late 1800s for several reasons: 1) Business leaders sought new overseas markets as trade barriers rose in Europe. 2) Social Darwinism and Anglo-Saxonism convinced many Americans they should dominate the world. 3) Growing European imperialism threatened US security. The Spanish-American War in 1898 marked the US emergence as a power, as the US defeated Spain and gained colonies like Puerto Rico and the Philippines, sparking a debate about imperialism.