Andrew Johnson was the 17th president of the United States, serving from 1865 to 1869. Johnson was Abraham Lincoln's vice president and so assumed the presidency after President Lincoln was assassinated.
Attempts were first made to impeach President Johnson in 1867, but it was not until 1868 that the House succeeded in impeaching him for violating the Tenure of Office Act, for the dismissal of Secretary of War Stanton, and for challenging congressional policies on Reconstruction. Johnson was acquitted by the Senate.
There are several books on Johnson’s presidency and impeachment that provide additional background information, some of which are shelved at the Law Library Reference Desk. The most useful of these may be Michael Les Benedict's The Impeachment and Trial of Andrew Johnson (1973), which provides an excellent critical bibliography on the topic.