 Following the Civil War, three constitutional amendments were passed, including the 13th Amendment (1865) that ended slavery; the 14th Amendment (1869) that gave African-Americans citizenship, adding their total population of four million to the official population of southern states for Congressional apportionment; and the 15th Amendment (1870) that gave African-American males the right to vote (only males could vote in the U.S. at the time).From 1865 to 1877, the United States underwent a turbulent Reconstruction Era trying to establish free labor and civil rights of freedmen in the South after the end of slavery.Many whites resisted the social changes, leading to insurgent movements such as the Ku Klux Klan, whose members attacked black and white Republicans to maintain white supremacy.