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.

On February 24, 1838, the two representatives and several other men met for a duel with rifles in Prince George’s County, Maryland. Neither congressman was very good with a rifle, and both missed each other or misfired on the first two rounds. On the third round, Graves shot and killed his colleague, Cilley.

This culture of violence also extended to state legislatures. The year before Graves killed Cilley, a representative in the Arkansas House insulted the Speaker during debate, and the Speaker responded by murdering him with a bowie knife right there on the House floor. “Expelled and tried for murder,” Freeman writes, “he was acquitted for excusable homicide and reelected, only to pull his knife on another legislator during debate, though this time the sound of colleagues cocking pistols stopped him cold.”

Congress responded to Cilley’s murder with an anti-dueling law in 1839, but the violence in Congress continued as its members led the U.S. into the Mexican-American War and fought over whether slavery should exist in new western territories. Brooks’ brutal attack on Sumner in 1856 was prompted by Sumner’s “Crime Against Kansas” speech that decried the South’s “Slave oligarchy” and demanded the U.S. admit Kansas as a free state. Brooks chose to beat Sumner rather than risk breaking the anti-dueling law because, he argued, dueling “would subject me to legal penalties more severe than would be imposed for a simple assault and battery.”

In 1858, partisan tensions over slavery erupted into a “full-fledged sectional combat on the floor,” Freeman writes. This was one year after the U.S. Supreme Court enraged abolitionists by ruling in Dred Scott v. Sanford that black people couldn’t be citizens and the federal government couldn’t ban slavery in western territories. The brawl started around 2:00 a.m. during an overnight session when a southern representative grabbed a northern representative by the throat and said he would teach the “black republican puppy” a lesson. As the two white men struggled, their colleagues ran over and a fistfight broke out.

“The end result was a free fight in the open space in front of the Speaker’s platform featuring roughly thirty sweaty, disheveled, mostly middle-aged congressmen in a no-holds-barred brawl, North against South,” Freeman writes.

Acts of violence like this showed how intensely southern congressmen wanted to preserve the economic, political and social power that they and their constituents held through owning slaves. They also presaged the larger fight between North and South that broke out three years later, when southern states seceded and declared war on the Union. After all, civil wars don’t just come out of nowhere.