The Senate had just adjourned on May 22, 1856, when Representative Preston Brooks entered its chamber carrying a cane. The pro-slavery southerner walked over to Senator Charles Sumner, whacked him in the head with the cane and then proceeded to beat the anti-slavery northerner unconscious. Afterward, Brooks walked out of the chamber without anyone stopping him.

The caning of Charles Sumner is probably the most famous violent attack in Congress, but it is far from the only one. In the three decades leading up to the Civil War, there were more than 70 violent incidents between congressmen, writes Yale history professor Joanne B. Freeman in The Field of Blood: Violence in Congress and the Road to the Civil War. It was a time of heightened tensions, especially over slavery—itself a violent institution that would drive the nation to a bloody war.

Congressmen during this period commonly carried pistols or bowie knives when they stepped onto the congressional floor. In fact, by the late 1850s, some constituents actually sent their congressmen guns. The fights that broke out among congressmen didn’t usually make it into newspapers (which themselves faced mob attacks for abolitionist sentiments); but there were some exceptions, especially in the decade before the Civil War. Brooks’ attack on Sumner, immortalized in a famous political cartoon, was one of those exceptions. Another was the only instance in which a congressman has ever killed another congressman.

That murder happened in 1838, when Congress was fiercely divided between the Whigs and the Democrats. At the time, many members considered an insult against a congressman to be an insult against his entire party. Challenging someone to a duel was therefore not just about a congressman’s own honor, it was also about defending the honor of his party. These were the circumstances under which representatives Jonathan Cilley and William Graves, who didn’t have any personal disagreement with each other, entered a duel that neither wanted.

It all started when Cilley, a Democrat from Maine, said something on the House floor that ticked off a prominent Whig newspaper editor. The editor asked Graves, a Whig from Kentucky, to hand-deliver a letter to Cilley asking if he wanted to take back what he’d said. But Cilley refused to accept the letter from the editor, who had a reputation for physically attacking congressmen, and Graves’ colleagues in the Whig party perceived this refusal as a slight. They advised Graves to challenge Cilley to a duel in order to maintain his political standing within his party. When Graves sent Cilley a letter challenging him to this duel, Cilley’s fellow Democrats told him he had to accept it for political reasons, too.