Andrew Johnson became president after the assassination of Abraham Lincoln in 1865.

His lenient Reconstruction policies clashed sharply with the Radical Republicans in Congress who wanted to protect the rights of freed slaves.

Johnson vetoed key civil rights legislation, including the Civil Rights Act of 1866 and the extension of the Freedmen's Bureau.

Congress overrode his vetoes, a first for major legislation in U.S. history.

To limit the president's power, Congress passed the Tenure of Office Act, which prevented him from firing cabinet members without Senate approval.

Johnson deliberately violated this act in 1868 by firing Secretary of War Edwin Stanton, an ally of the Radicals.

In response, the House of Representatives voted to impeach Johnson for 'high crimes and misdemeanors.'

He was the first U.S. president to be impeached.

His trial was held in the Senate, which had the power to remove him from office with a two-thirds vote.

Johnson was acquitted by a single vote, and he served the remainder of his term.