Former President, Abraham Lincoln was shot and killed in a theater. The vice-president, Andrew Johnson, became president.
Right away, when Andrew Johnson was in office, people didn't like what he was doing, especially the Radical republicans.
President Andrew Johnson
The president then issues the Proclamation of Amnesty, when the Congress was not is session.
The president continues to now, veto bills, and he continues to do things when Congress is not in session. In almost all of these, Congress overrides whatever the president did.
This was almost like the last straw for Congress. The president now removes the sectary of war, the senate did not consent, and he did it anyway. This was one of many violations. The filled him for impeachment.
The senate began the first ever impeachment of a U.S. president. The judge eventually found Andrew Johnson not guilty, by only one vote.