At first it just seemed like a longwinded speech on the floor of the U.S. Senate—a five-hour, 112-handwritten-page address delivered over the course of two days in May 1856. But Charles Sumner, a senator for Massachusetts, had no way of knowing that “The Crime Against Kansas,” his fiery soliloquy that spoke out on the behalf of disenfranchised slaves, would become one of American history’s most inflammatory—and dangerous—speeches.

Sumner’s target was the Kansas-Nebraska Act, which gave citizens of the newly created territories “popular sovereignty,” and the right to vote for or against slavery. Sumner found the new law tantamount to “the rape of a virgin Territory,” and targeted several Southern senators, including Andrew Butler, for an extra dose of his scorn.

Butler, a pro-slavery senator from South Carolina, was absent that day and unable to defend himself. Nonetheless, Sumner decried Butler’s position on slavery. He mocked his notion of chivalry, saying “he has chosen a mistress to whom he has made his vows, and who, though ugly to others, is always lovely to him; though polluted in the sight of the world, is chaste in his sight—I mean the harlot, Slavery.”

Sumner was the rare northerner who combined an anti-slavery stance with abolitionism and an absolute conviction in equal rights. Before he began his career in politics, he worked as a lawyer on a number of cases concerning African-American rights. In 1843 he opposed a state law that prohibited interracial marriage; in 1849 he represented the young Sarah Roberts, an African-American girl, in a school segregation case. Upon entering Congress, his first memorable speech was “Freedom National,” in which he critiqued the Fugitive Slave Act.

The Massachusetts senator’s provocative language and firm stance made him so unpopular that he was regularly jeered by other senators, denied the floor, and blocked from participating in congressional committees. But Sumner’s vocal advocacy for African-Americans didn’t go unnoticed. Shortly after making his “Freedom National Speech,” Frederick Douglass wrote him an encouraging letter. “All the friends of freedom, in every state, and of every color, may claim you, just now, as their representative. As one of your sable constituents—My dear Sir, I desire to thank you, for your noble speech for freedom.”

But Sumner’s popularity with the abolitionist crowd only made him more despicable in the eyes of Southern plantation owners—especially Preston Brooks, a congressman from South Carolina and the second cousin of the insulted Butler. Two days after Sumner’s speech, Brooks attacked him with a cane at his desk in the Old Senate Chamber after the day’s session had adjourned. Sumner, trapped beneath his desk, was soon dripping with blood and fell unconscious. Assisting Brooks in the attack was South Carolina congressman Laurence Keitt, who had a reputation for histrionics. Keitt fended off shocked onlookers until finally several were able to rush forward and carry Sumner out. The cane Brooks used for the attack had shattered. He pocketed its gold head and left the building.

“Every lick went where I intended,” Brooks bragged after the attack. “For about the first five of six licks he offered to make flight but I plied him so rapidly that he did not touch me. Towards the last he bellowed like a calf,” he wrote.

The calculated attack was meant to impart a very particular message. Brooks could have challenged challenging Sumner to a duel—he had already participated in two at that point. Instead, he chose to attack his colleague with a cane—a weapon that in other circumstances would have been used to punish a slave.

The attack left two bone-deep cuts on Sumner’s head and bruises on his head, shoulders and hands. Though the doctor who first attended Sumner thought he might recover quickly from the wounds, infection soon set in. It would be four years before Sumner was able to return to his place in the Senate. The state of Massachusetts re-elected him and left his seat empty for the entire period as a reminder of the brutal attack.

“Southerners had declared abolitionists miscreants and criminals,” says Manisha Sinha, author of The Slave’s Cause: A History of Abolition and professor of history at the University of Connecticut. “There were laws in Southern states that said you could be imprisoned for speaking out against slavery, so to have someone like Sumner speaking out in Congress really galled them.”

A District court fined Brooks $300, which his supporters in the South paid. Though multiple members of the House called for Brooks’ expulsion, they couldn’t garner enough votes. They censured Keitt instead. Both Brooks and Keitt then resigned in protest of their treatment—and were later reelected. (Keitt later left the House of Representatives again to join the Confederacy.)

“If I desired to kill the senator why did I not do it? You all admit that I had him in my power. It was expressly to avoid taking life that I used an ordinary cane,” said Brooks in an unapologetic resignation speech. He argued that he only meant to insult Sumner, not government institutions, and that to even consider expelling him was an affront.

Though Congress’ reaction was relatively subdued, the incident’s impact on the country at large was enormous. Southern newspapers and plantation owners extolled Brooks for putting Sumner in his place; Northerners praised Sumner and vilified Brooks.

The attack incited hundreds of “indignation meetings” across the North—the 19th-century equivalent of Internet hashtag outrage. With a history that stretched back to the Stamp Act protests, the meetings gave citizens a formalized, non-partisan way of expressing their reactions. In New York, the indignation meeting at Broadway Tabernacle drew a crowd of more than 5,000, all eager to express their shock over Sumner’s treatment. Seeing the effectiveness of these gatherings in harnessing public opinion, the nascent Republican Party organized their own gatherings that adhered closely to the format of indignation meetings.

“Although northern voters never achieved complete unanimity, indignation meetings did encourage political unity throughout the free states, indicating to numerous observers that ‘the North’ had emerged as a potent political entity,” wrote historian Michael Woods in the Journal of Social History.

For Sinha, there are disturbing echoes between the political antagonism of the antebellum period and today—especially given the recent rebuke of Senator Elizabeth Warren after she read a 1986 letter written by the widow of Civil Rights leader Martin Luther King in opposition to Senator Jeff Sessions’ nomination as U.S. Attorney General. Her recitation was cut short when Senate Majority Leader Mitch McConnell invoked a century-old rule that forbids impugning a fellow senator’s character. Like Sumner, Sinha sees Warren as using her position of power to give voice to the disenfranchised—and being punished for doing so. “Senator McConnell saying, ‘She was warned and yet she persisted,’ [of Warren] is very representative of men rebuking abolitionist women who spoke out against slavery,” Sinha says. “Things have changed, but many times we hear similar ways in which women or African-Americans are being written out of the body politic.”

In the case of Sumner and Brooks, the caning was more than an isolated instance of violence; it was a rallying cry for northerners determined to protect republicanism, democracy and freedom of speech. “Bloody Sumner” was soon tied up with “Bloody Kansas,” offering the early Republican Party proof that the South meant to extinguish the liberties of the North. The attack deepened that rift—and would go on to have deadly consequences for the country.

“From the perspective of white southerners, the decision to embrace brute force did not end well,” write historians James Hill Wellborn and Stephen Berry. “By 1865, they had lost 25 percent of their military-aged men. Their war to secure slavery had destroyed it instead.”

It might be a lesson worth revisiting in the modern era, thinks Sinha.

“The more you overreach, the more you’ll have reaction against you,” she says, referencing the sudden popularity of Corretta Scott King’s letter and the backlash to McConnell’s rebuke. She finds the current furor reminiscent of the uproar that followed Sumner’s beating, and hopes history can be used as a lesson to prevent the political rift from growing. “It took a war to abolish slavery. I hope it doesn’t take a war to get us back on track to democratic norms.”

Editor's note (February 13, 2017): A previous version of the headline implied both Brooks and Sumner were senators; only Sumner was a senator, while Brooks was a member of the House of Representatives