In order to warrant articles of impeachment being brough against him, he defied the Tenure of Office Act by firing a cabinet member. Although the impeachment trial of Andrew Johnson was ostensibly about a violation of the Tenure of Office Act, it was about much more than that. Also on trial in 1868 were Johnson's lenient policies towards Reconstruction and his vetoes of the Freedmen's Bureau Act and the Civil Rights Act. The trial was, above all else, a political trial. I hope this is a god info for you