Timeline of the impeachment of Andrew Johnson
Andrew Johnson became the first president of the United States to be impeached by the United States House of Representatives on February 24, 1868 after he acted to dismiss Edwin Stanton as secretary of war in disregard for the Tenure of Office Act. Before Johnson's February 21, 1868 effort to dismiss Stanton, there had already been an active push by Radical Republicans to impeach Johnson, but this was met with resistance from many in the moderate wing of the Republican Party.
Source: Wikipedia — Timeline of the impeachment of Andrew Johnson (CC BY-SA 4.0)