The post-Civil War decades, known as the Gilded Age, brought rapid economic growth, industrialization, and the rise of powerful figures such as John D. Rockefeller in oil, Andrew Carnegie in steel, and J.P. Morgan in finance, men critics called "Robber Barons" for their monopolistic practices and defenders called "captains of industry." Industrialization brought stark inequality, urbanization, and political corruption, sparking the Progressive Era of the 1890s through the 1920s. Progressives pushed antitrust laws, food safety regulations, direct election of senators, and women's suffrage, culminating in the Nineteenth Amendment (1920), which granted women the right to vote after decades of activism by figures like Susan B. Anthony and Elizabeth Cady Stanton. Abroad, the United States emerged as a colonial power after its swift victory in the 1898 Spanish-American War, gaining Puerto Rico, Guam, and the Philippines, while President Theodore Roosevelt advanced the Panama Canal (completed 1914) and a "Big Stick" foreign policy.
When World War I erupted in Europe, the United States initially remained neutral but entered in April 1917 after German unrestricted submarine warfare and the Zimmermann Telegram. American troops and resources tipped the balance in favor of the Allies, leading to the November 1918 armistice under President Woodrow Wilson. The 1920s roared with jazz music, flappers, mass consumerism, Prohibition (1920 to 1933) under the Eighteenth Amendment, and the Harlem Renaissance, a flourishing of African American literature, music, and art led by figures like Langston Hughes, Zora Neale Hurston, and Duke Ellington. The decade ended abruptly with the October 1929 stock market crash, which, compounded by bank failures, overproduction, and weak agricultural prices, triggered the Great Depression. By 1933, unemployment reached 25 percent, and the Dust Bowl ravaged the Great Plains, displacing hundreds of thousands of "Okies" who migrated westward in search of work.
President Franklin D. Roosevelt responded with the New Deal (1933 to 1939), a sweeping program of reforms including the Civilian Conservation Corps, Works Progress Administration, Social Security Act, and Securities and Exchange Commission, fundamentally expanding the federal government's role in the economy. The Japanese attack on Pearl Harbor on December 7, 1941, killed over 2,400 Americans and drew the United States into World War II. Following Pearl Harbor, Executive Order 9066 forced approximately 120,000 Japanese Americans into internment camps, a grave civil liberties violation later formally acknowledged and redressed. The 1944 G.I. Bill provided returning veterans with college tuition, low-cost home loans, and unemployment insurance, fueling the expansion of the middle class and suburban growth. President Harry S. Truman authorized the atomic bombings that ended the war with Japan, implemented the Marshall Plan to rebuild Europe, and desegregated the military in 1948.