Through the 19th century, America concentrated on creating a nation that spanned the continent, and it avoided foreign entanglements. Once industrialized and more prosperous, it began looking for foreign markets and colonies.
By the turn of the 20th century, the United States had become a minor imperial power, fighting a war with Spain for Cuba and the Philippines and annexing Hawaii and several other territories. World War I engaged the United States in European affairs, but after the war, a wave of isolationist feeling swept the country. Refusing membership in the League of Nations, America turned inward once again. Absorbed by the prosperity of the 1920s and the Great Depression of the 1930s, America let its military strength erode.