As the United States grew and expanded into new territories throughout the early to mid-19th century, divisions also deepened. At the heart of those divisions lay the issue of slavery. In 1846, a Pennsylvania representative named David Wilmot proposed legislation intended to bar slavery’s expansion into the lands acquired after the Mexican-American War.

The Wilmot Proviso failed to pass—and debate over the proposal exacerbated North-South tensions. “It is part of a larger, broader discussion about the future of slavery,” says Dr. Miller W. Boyd III, a historian, teacher and lecturer in St. Louis, Missouri. “It is one small link in a long chain going towards the Civil War.”

Slavery emerged as a hotly contested issue between the North and the South in large part because different economic realities prevailed in the different regions. The North’s economy was focused more on industry and manufacturing, while crops like tobacco and cotton were big money-makers for white Southerners.

“In Pennsylvania, for example, people were growing wheat. Wheat was not as labor-intensive as other cash crops. Slaves were almost incidental to society,” Boyd explains. “In a place like Louisiana or Virginia, you don't have a society with slaves, you have a slave society. Everything is determined or shaped around chattel slavery.”

Those differences in economic priorities influenced how slavery was viewed and the degree to which it was tolerated or considered necessary in various parts of the country.

Early Abolitionists tended to be Northerners who objected to slavery on religious and moral grounds. They recognized the humanity of enslaved people, the inhumanity of slavery, and saw the contradiction in allowing such a racist system to persist in a country founded on the principle that “all men are created equal.”

Southerners benefited exponentially from the economic advantages of keeping a compulsory work force in bondage and most were unwilling to see slavery abolished. They resented attempts to curtail it—arguing that slavery was the life’s blood of their economy and an integral part of their way of life. That stance paved the way to the Confederacy and its eventual secession from the United States.

Wilmot wasn’t an abolitionist or an advocate for slavery. He and those affiliated with him, as members of the Free Soil Party, weren’t fighting to preserve or abolish slavery. They were content to let slavery persist, however they wanted to prevent it from spreading as America expanded into new territory. Wilmot and those aligned with him wanted to create and preserve economic opportunities for white citizens.

“Wilmot wanted more land available for poor and middling whites to cultivate,” says Boyd. “He was trying to help them not have to compete everywhere with slaved-produced crops.” Because wherever slavey was allowed to thrive, it took jobs from and suppressed the wages of the working class. Wilmot and his affiliates also feared that slave states had gained too much political power and wanted to see that influence curtailed.

So when, in August of 1846, President James Polk put forward a special appropriations bill to ask Congress for $2,000,000 to acquire territory from Mexico as part of peace negotiations after the Mexican-American War, Wilmot’s Proviso sought to keep slavery from expanding into any of that newly obtained land.

“This war was fought for the acquisition of territory and the hope of expanding not only American borders, but slavery as well,” says Boyd.

Wilmot was a member of a Democratic Party that had split into opposing factions over the issue of slavery during the 1844 election. That rift became further entrenched when President Polk accepted less land in a compromise with Great Britain involving Oregon yet sought a greater percentage of Texas from Mexico.

Northern Democrats like Wilmot worried about the implications of the United States acquiring additional territory where slavery would be permitted. By sponsoring the Wilmot Proviso, Wilmot was acting on behalf of his constituents, namely free, white Pennsylvanians.

(See the full text of Wilmot’s Proviso as preserved by the National Archives.)

The Wilmot Proviso was dead in three days. It passed twice in the U.S. House of Representatives where Northerners had the majority. But it failed in the U.S. Senate where there was an equal amount of support for the free states and the slave states. Polk’s “Appropriation to Secure Peace Bill” passed in early 1847 without Wilmot’s Proviso.

Although the Wilmot Proviso of 1846 didn’t pass with Polk’s appropriations bill, it still left a lasting legacy. The proposed amendment’s purpose was simple and straightforward: slavery and involuntary servitude would be banned forever in all territories acquired as a result of the Mexican-American War. The only exception (much like in the 13th Amendment) would be unpaid labor as punishment for a crime.

The similarity in the wording of those two documents is not coincidental. The 13th Amendment borrowed some of its language from the Wilmot Proviso, which in turn took some of its language from the Northwest Ordinance of 1787.

Although Wilmot’s Proviso failed by itself, it fueled volatile division and debate between the Northern and Southern states over slavery and ultimately made the Civil War more inevitable. It also spurred on the creation of the Republican Party because, as Boyd explains, “the Republican party was the culmination of this anti-slavery, abolitionist, but also mostly Free Soil rhetoric.”

The Wilmot Proviso highlighted competing economic interests that inflamed North-South tensions around the topic of slavery, pushed the country closer to the Civil War, and continued to divide America well after the ratification of the 13th Amendment.

Wilmot's Proviso, UShistory.org.

The Wilmot Proviso, NBC News Learn.

Wilmot Proviso Transcript, U.S. National Archives.

"How are U.S. states, territories, and commonwealths designated in the Geographic Names Information System?" USGS.