From Wikipedia: The bloody American Civil War (1861-5) redefined the nation and remains the central iconic event. The South was defeated and, in the Reconstruction era, the U.S. ended slavery, extended rights to African Americans, and readmitted secessionist states with loyal governments. The national government was much stronger, and it now had the explicit duty to protect individuals. Reconstruction was never completed by the US government and left the blacks in a world of Jim Crow political, social and economic inferiority. The entire South remained poor while the North and West grew rapidly. ~~~ Thanks to an outburst of entrepreneurship in the North and the arrival of millions of immigrant workers from Europe, the U.S. became the leading industrialized power by 1900. Disgust with corruption, waste, and traditional politics stimulated the Progressive movement, 1890-1920s, which pushed for reform in industry and politics and put into the Constitution women's suffrage and Prohibition of alcohol (the latter repealed in 1933). Initially neutral in World War I, the U.S. declared war on Germany in 1917, and funded the Allied victory. The nation refused to follow President Woodrow Wilson's leadership and never joined the League of Nations. After a prosperous decade in the 1920s the Wall Street Crash of 1929 marked the onset of the decade-long world-wide Great Depression. A political realignment expelled the Republicans from power and installed Democrat Franklin D. Roosevelt and his elaborate and expensive New Deal programs for relief, recovery, and reform. Roosevelt's Democratic coalition, comprising ethnics in the north, labor unions, big-city machines, intellectuals, and the white South, dominated national politics into the 1960s. After the Japanese attack on Pearl Harbor in December 1941, the U.S. entered World War II alongside the Allies and helped defeat Nazi Germany in Europe and, with the detonation of newly-invented atomic bombs, Japan in Asia and the Pacific.