Why was Johnson impeached?
The U.S. House of Representatives vehemently felt that President Johnson was not doing his job properly. They initially attempted to impeach him in November 1867 for a variety of reasons, but the vote failed 57 to 108. However, the House found a new reason to impeach President Johnson after he removed Secretary of War Edwin Stanton from office in violation of a newly enacted federal law known as the Tenure of Office Act.
The Tenure of Office Act prohibited the president from discharging members of his cabinet until a successor had gone through official Senate approval. Congress had passed the law in large measure to protect Stanton, whom Johnson wanted out of office. Johnson ignored the Tenure of Office of Act and had Stanton—who had barricaded himself in his office—removed.
A few days later the House impeached President Johnson. Under the Constitution, the House can impeach a president (and other federal officials), but the Senate has the power to try and convict the person. The Senate has to vote by a two-thirds margin to actually remove the person from office via impeachment. The Senate voted thirty-five to nineteen that Johnson was guilty and should be impeached. This was one vote shy of the necessary two-thirds majority. Thus, Johnson survived the impeachment process and remained in office.