About 150 years ago, as Congress prepared to impeach President Andrew Johnson, someone discovered two bottles of what seemed like nitroglycerin in a Senate passageway. Hysterical politicians fled the building—until some bold newspaperman swigged the liquid. It was just bourbon.

The incident provided a powerful metaphor for the way impeachment turned a substance politicians could usually handle into something highly explosive. After months of drama, Lincoln’s former secretary John Hay concluded: “Impeachment is demonstrated not to be an easy thing. The lesson may be a good one some day.” The lesson, as good in 2017 as it was in 1868, is that removing a president is an ugly process, which can dangerously inflame tensions in an already divided nation.

There were, in the late 1860s, real fears that impeachment could spark a second Civil War. That rebellion was barely over, and posed a number of unanswered questions. What did the nation owe to millions of freed slaves? How should the federal government treat Confederate leaders and seceded states? What should northerners do about the atrocious outbreaks of racist violence unfolding in cities like New Orleans and Memphis?

Things were little calmer in Washington. A victorious, sometimes-cocky, Republican Party controlled more than three-quarters of both houses of Congress. Yet in the White House sat Andrew Johnson, put into power not by a popular vote, but by Lincoln’s assassination. And Johnson, it was painfully clear, was hostile to blacks, lenient with rebels, and hell-bent on fighting Congress.

Johnson was, possibly, the worst man to lead the country at such a tense moment. Racist, crude, and grumpy, Johnson nursed an incredible persecution complex. At best, he was a formerly illiterate tailor who had worked his way up from poverty to the most powerful position in the nation, like his fellow Tennessean and personal hero, Andrew Jackson. At worst, he was paranoid, resentful, narcissistic. Washington politicos described a man who “always hated somebody,” “always defeats himself,” and was “always worse than you expect.”

There were, still, millions who sided with Johnson. White Democrats, especially in the lower north and the south, felt overwhelmed by Republicans. To them, Republicans were social-justice warriors intent on revolutionizing race relations and centralizing Federal power; most Democrats just wanted to return to the old union and old Constitution. Such Democrats launched the most bitterly racist campaigns in American history, rallying behind Andrew Johnson as a symbol of their struggle against change.

Johnson put his presidency on a collision course with Congress. He referred, constantly, to his unfair treatment and to his many enemies, at whom he spat bold, baseless claims. After white police officers slaughtered dozens of black activists in New Orleans, Johnson ridiculously blamed “the radical Congress” for the massacre. On a disastrous speaking tour across the Midwest—in which he drank heavily and compared himself to Jesus—Johnson called for the lynching of his most hated congressional rival, Thaddeus Stevens. Even relatively neutral observers like Senator John Sherman eventually concluded that he was beyond help, sighing: “The truth is, he is a slave to his passions and resentments.”

Johnson’s enemies were nearly as intent on conflict. They set a trap for the president, making it illegal for him to fire certain officials. Of course, Johnson promptly fired them. His nemesis, Stevens, was heard hollering in Congress: “Didn’t I tell you so? If you don’t kill the beast, it will kill you.” Congress moved in, voting for impeachment in March 1868.

–Jon Grinspan, The High Price of Presidential Impeachment