When Abraham Lincoln said “a house divided against itself cannot stand,” he wasn’t talking about the kind of political divisions common today. Americans may differ sharply on issues like immigration and abortion, but there is no single issue that geographically and economically divides the country in the same way that slavery did in the 1850s. Back then, the U.S. was so divided that many feared it would break out in civil war—a fear that Lincoln unwittingly stoked.

Lincoln’s now-famous “house divided” line, which is drawn from the Bible, was actually part of a campaign speech he delivered at the 1858 Illinois Republican State Convention. Lincoln, then a relatively unknown politician, had just won the nomination to run for U.S. Senate against one of the most important politicians in the country, Stephen A. Douglas.

In general, Democrats then were the party of the slave-holding south and Republicans were the party of the free north that opposed slavery’s expansion. Yet Republicans weren’t too concerned about Lincoln’s race because they thought Senator Douglas, a Democrat, might be open to working with them against expanding slavery.

“Douglas had been seeking a middle ground between North and South, some way of comprising on the slavery issue,” says Eric Foner, a history professor at Columbia University who has written several books about slavery and the Civil War.

In his “house divided” speech, Lincoln countered that the Dred Scott decision the previous year had already opened the doors for slavery to be legal in the north, as well as all territories that the U.S. expanded into. If the U.S. wanted to be a free country, he argued, it had to act now before it was too late.

“Lincoln’s saying, ‘No, there is no compromise,’” Foner explains. “‘You’ve got to be on one side or the other.’ In effect, he’s saying, ‘I’m on the side of freedom and Douglas … is on the side of slavery.’” Or, to quote the man himself: “A house divided against itself cannot stand. I believe this government cannot endure, permanently half slave and half free. I do not expect the Union to be dissolved—I do not expect the house to fall—but I do expect it will cease to be divided. It will become all one thing or all the other.”

The speech certainly got Republicans’ attention. Many thought it had done too much, says Allen Guelzo, director of Civil War era studies at Gettysburg College and author of multiple books about Lincoln.

“As soon as he used those words ‘house divided,’ he articulated the fear that everybody had at that point that the slavery controversy was indeed going to lead to some kind of civil war,” Guelzo says. “And you could almost hear a collective gasp from people that he would actually come right out and say we’re going to have a civil war.”

To be clear, this wasn’t what Lincoln was saying. He thought the country couldn’t remain half-free, and that it would end up becoming one or the other. But in the very next sentence, he clarified that he didn’t think this would necessarily happen through the dissolution of the Union.

“Of course, nobody paid attention to the second sentence,” Guelzo says. “All they heard was the ‘house divided’ part and they immediately assumed that what Lincoln was calling for was civil war, that the only way to resolve the slavery injustice was going to be civil conflict.” On the campaign trail, Douglas used the perception that Lincoln was advocating for war against him.

“I think that there’s a fairly good argument to be made that the ‘house divided’ speech ended up hurting Lincoln in the 1858 election and was one reason why he lost,” Guelzo says. “Because it made him sound too unacceptably radical on the subject of slavery, even to people who were opposed to the extension of slavery … but they really didn’t wanna have a civil war over it.”

Still, Lincoln emerged from the campaign as a prominent political figure. Not many people heard about his “house divided” address when he first gave it, but over the next several months, he publicly spread his argument through campaign speeches and his famous series of debates with Douglas.

The national attention Lincoln gained from that campaign earned him the Republican presidential nomination and then the presidency in 1860. Soon after this victory, his “house divided” speech became strangely prophetic as southern states seceded from and waged war on the Union. In 1865, the U.S. accomplished Lincoln’s goal of abolishing slavery—but not without fighting a civil war over it first.