why did andrew johnson get impeached

Andrew Johnson was impeached primarily because he violated the Tenure of Office Act by removing Edwin Stanton, the Secretary of War, from his position without Senate approval and appointing Lorenzo Thomas as the new Secretary of War ad interim. The Tenure of Office Act, passed by Congress over Johnson's veto, was designed to protect Stanton, a Radical Republican ally, from being dismissed without Senate consent. Johnson's action was seen as a direct challenge to congressional authority and its Reconstruction policies. Additional charges included allegations of inflammatory speeches and attempting to undermine congressional Reconstruction efforts. Despite the House impeaching him, Johnson was acquitted in the Senate by one vote short of the needed two-thirds majority for conviction.