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The Impeachment came about as the result of a lengthy political battle, between Johnson and the "Radical Republicans" in Congress for control of Reconstruction policies after the Civil War. The Radical Republicans were convinced that as President, Johnson would enact their hardline Reconstruction policies of protection for newly freed slaves and punishment for former slave owners and government and military officials. Lincoln had favored a much more moderate and lenient plan for Reconstruction, which the Radicals vehemently opposed but lacked the political support to prevent. Six weeks after taking office, Johnson announced a policy of general amnesty for most former confederates, and also vetoed legislation that extended civil rights and financial support for the former slaves. Congress was able to override only a few of his vetoes, setting the stage for a battle between Congress and the president.
Johnson was impeached on February 24, 1868 in the U.S. House of Representatives on eleven articles of impeachment. Article Two of the Constitution calls for "high crimes and misdemeanors" in order to impeach a president. The House's primary charge against Johnson was with violation of the Tenure of Office Act. Specifically, he had removed Edwin M. Stanton, the Secretary of War (whom the Tenure of Office Act was largely designed to protect), from office and replaced him with Ulysses S. Grant.
The House agreed to the articles of impeachment on March 2, 1868.
Johnson's trial began three days later in the Senate, with Supreme Court Chief Justice Salmon P. Chase presiding. (Chase had been Lincoln's first Secretary of the Treasury.) The trial concluded on May 26 with Johnson's acquittal, the final count falling one vote shy of the required tally for conviction.
The impeachment and subsequent trial have been considered by many historians a politically motivated, rather than justified. Until the impeachment of President Bill Clinton 131 years later, it was the only presidential impeachment in the history of the United States.