With a decisive victory in 1865 over Southern secessionists in the Civil War , the United States became a united and powerful nation with a strong national government . Reconstruction brought the end of legalized slavery plus citizenship for the former slaves , but their new-found political power was rolled back within a decade , and they became second-class citizens under a " Jim Crow " system of deeply pervasive segregation that would stand for the next 80 – 90 years . Politically , during the Third Party System and Fourth Party System the nation was mostly dominated by Republicans ( except for two Democratic presidents ) . After 1900 and the assassination of President William McKinley , the Progressive Era brought political , business , and social reforms ( e.g. , new roles for and government expansion of education , higher status for women , a curtailment of corporate excesses , and modernization of many areas of government and society ) . The Progressives worked through new middle-class organizations to fight against the corruption and behind-the-scenes power of entrenched , state political party organizations and big-city " machines " . They demanded — and won — women 's right to vote , and the nationwide prohibition of alcohol 1920-1933
assassination of President William McKinley
Assassination of William McKinley
History of the United States (1865–1918)