The underlying reason for the impeachment of Andrew Johnson, the 17th President of the United States, was his clash with the Radical Republicans in Congress over the Reconstruction policies following the American Civil War. The main issue at the center of Johnson's impeachment was his resistance to implementing the civil rights agenda and the equal rights for freed slaves that the Radical Republicans advocated for. This clash led to political tensions and disagreements, eventually resulting in Johnson's impeachment by the House of Representatives in 1868.