Thirteenth Amendment to the United States Constitution
Near the end of the Civil War, Republicans, who controlled Congress, introduced an amendment to make slavery illegal (against the law) in every part of the United States. (In order for an amendment to happen, it first must be passed by both the United States Senate and the United States House of Representatives, and then passed by three-quarters of the states' legislatures.)  The amendment was passed by three-quarters of the states, and became law in late 1865.