Jackson had been sworn in as president after the assassination of Lincoln in April 1865. He was in favor of a very lenient reconstruction policy in South following the Civil War, which won him many enemies in the Republican dominated Senate.

Jackson stood accused of violating the controversial Tenure of Office Act which had been passed by the Senate to curb Jackson's powers to remove cabinet members appointed under Lincoln.

In the fall of 1867, Johnson attempted to test the constitutionality of the act by replacing Edwin M. Stanton with General Ulysses S. Grant as secretary of war. The Supreme Court refused to rule on the case, however, and Grant turned the office back to Stanton after the Senate passed a measure in protest of his appointment.

On February 21, 1868, Johnson decided to challenge the Senate: he appointed General Lorenzo Thomas, an individual far less favorable to the Congress than Grant, as secretary of war. Stanton refused to give up his post, he barricaded himself into his office. The House of Representatives, which had already discussed impeachment after Johnson's first dismissal of Stanton, initiated formal impeachment proceedings against the president.

Jackson's opponents were unable to get the two-thirds majority required to impeach him (by one vote).