Only two American presidents have ever endured an impeachment trial: Andrew Johnson, and Bill Clinton. One other president, Richard M. Nixon, resigned from office before the terms of his impeachment could be drawn up.
Andrew Johnson was Abraham Lincoln's Vice President. After Lincoln was assassinated, Johnson became President. Those were strange times. Lincoln was a Republican. Johnson had been the only southern U.S. Senator who opposed the secession by the southern states which led to the Civil War. Johnson was a southern Democrat. He didn't have any major quibbles with slavery.