This is the ‘official story’ from Wikipedia:
The impeachment of Andrew Johnson occurred in 1868, when the United States House of Representatives resolved to impeach U.S. President Andrew Johnson, adopting eleven articles of impeachment detailing his “high crimes and misdemeanors”, in accordance with Article Two of the United States Constitution. The House’s primary charge against Johnson was violation of the Tenure of Office Act, passed by the U.S. Congress in March 1867, over the President’s veto. Specifically, he had removed from office Edwin McMasters Stanton, the Secretary of War—whom the Act was largely designed to protect—and attempted to replace him with Brevet Major General Lorenzo Thomas. (Earlier, while the Congress was not in session, Johnson had suspended Stanton and appointed General Ulysses S. Grant as Secretary of War ad interim.)
The House approved the articles of impeachment on March 2–3, 1868, and forwarded them to the Senate. The trial in the Senate began three days later, with Chief Justice of the United States Salmon P. Chase presiding. On May 16, the Senate failed to convict Johnson on one of the articles, with the 35–19 vote in favor of conviction falling short of the necessary two-thirds majority by a single vote. A ten-day recess was called before attempting to convict him on additional articles. The delay did not change the outcome, however, as on May 26, it failed to convict the President on two articles, both by the same margin; after which the trial was adjourned.
This was the first impeachment of a President since creation of the office in 1789. The culmination of a lengthy political battle between Johnson, a lifelong Democrat and the Republican majority in Congress over how best to deal with the defeated Southern states following the conclusion of the American Civil War, the impeachment, and the subsequent trial (and acquittal) of Johnson were among the most dramatic events in the political life of the nation during the Reconstruction Era. Together, they have gained a historical reputation as an act of political expedience, rather than necessity, which was based on Johnson’s defiance of an unconstitutional piece of legislation, and which was conducted with little regard for the will of a general public which, despite the unpopularity of Johnson, opposed the impeachment.
Johnson is one of only three presidents against whom articles of impeachment have been reported to the full House for consideration. In 1974, during the Watergate scandal, the House Judiciary Committee approved articles of impeachment against Richard Nixon, who resigned from office, rather than face certain impeachment and the prospect of being convicted at trial and removed from office. In 1998, Bill Clinton was impeached; he, like Johnson, was acquitted of all charges following a Senate trial.
In episode 35 of Myths VS Facts Art Thompson, CEO of the John Birch Society, discusses how President Andrew Johnson, a flawed patriot, tried to expose the Conspiracy’s plan. Find out what President Johnson did and the backlash he received.
By mid-1867, Johnson’s enemies in Congress were repeatedly promoting impeachment. The precipitant event that resulted in a third and successful impeachment action was the firing of Secretary of War Edwin M. Stanton.