第一次世界大战使欧洲元气大伤，美国却因置身事外，损失不多，声望大为增加。在整个1920年代，农产品价格不断降低，工业利润大幅增长，美国经济发展得异常繁荣。
After the Gilded Age came the Progressive Era, whose followers called for reform over perceived industrial corruption. Viewpoints taken by progressives included greater federal regulation of anti-trust laws and the industries of meat-packing, drugs, and railroads. Four new constitutional amendments—the Sixteenth through Nineteenth—resulted from progressive activism. The era lasted from 1900 to 1918, the year marking the end of World War I.
U.S. Federal government policy, since the James Monroe Administration, had been to move the indigenous population beyond the reach of the federal frontier into a series of Indian reservations. Tribes were generally forced onto small reservations as farmers and ranchers took over their lands.
The United States began its rise to international power in this period with substantial population and industrial growth domestically and numerous military ventures abroad, including the Spanish-American War, which began when the United States blamed the sinking of the USS Maine on Spain. Also at stake were U.S. interests in acquiring Cuba, an island nation fighting for independence from Spanish occupation; Puerto Rico and the Philippines were also two former Spanish colonies seeking liberation. In December 1898, representatives of Spain and the U.S. signed the Treaty of Paris to end the war, with Cuba becoming an independent nation and Puerto Rico, Guam, and the Philippines becoming U.S. territories. In 1900, Congress passed the Open Door Policy that at the time required China to grant equal trading access to all foreign nations.
President Woodrow Wilson declared U.S. entry into World War I in April 1917 following a yearlong neutrality policy; the U.S. had previously shown interest in world peace by participating in the Hague Conferences. American participation in the war proved essential to the Allied victory. Wilson also implemented a set of propositions titled the Fourteen Points to ensure peace, but they were denied at the 1919 Paris Peace Conference. Isolationist sentiment following the war also blocked the U.S. from participating in the League of Nations, an important part of the Treaty of Versailles.