Introduced by Democratic Congressman David Wilmot of Pennsylvania, the Wilmot Proviso would have banned the extension of slavery into any territories Mexico ceded to the U.S. after the Mexican-American War. Although never enacted by Congress, the Wilmot Proviso hastened the division of the American political landscape along sectional lines, ultimately contributing to the onset of the Civil War.
The Constitutional Convention
When delegates to the Constitutional Convention assembled in Philadelphia in 1787, one of the more daunting tasks that they faced was resolving sectional differences between the North and South centered on slavery. After weeks of debate proved futile, the delegates negotiated a series of compromises that enabled them to proceed with their primary assignment of forming “a more perfect Union” between the separate states. In the short term, the compromises regarding the status of slavery established in the Constitution facilitated the creation of the new republic (at the expense of blacks held in bondage), but they also sowed the seeds of turmoil that began coming to fruition as the nation expanded west in the coming decades.
The Northwest Ordinance
As the delegates to the Constitutional Convention set about creating a new government, representatives to the Congress of the existing government established under the Articles of Confederation were meeting in New York. On July 13, 1787, the Confederation Congress enacted “An Ordinance for the government of the Territory of the United States northwest of the River Ohio,” that stipulated “There shall be neither slavery nor involuntary servitude in the said territory. . . .” That legislation, more commonly known as the Northwest Ordinance, had the effect of establishing the Ohio River as the border separating free and slaves states between the Appalachian Mountains and the Mississippi River. For the next three decades, that boundary forestalled major sectional disputes over slavery.
The Louisiana Purchase
Circumstances changed in 1803 when Napoleon Bonaparte sold President Thomas Jefferson 828,000 square miles of land west of the Mississippi River. In hindsight, the Louisiana Purchase may be viewed as a logical progression of the American Westward Movement, but at the time, it created new challenges for the federal government. Besides land ownership issues regarding the native inhabitants, the Louisiana Purchase eventually forced Congress to address the subject of slavery in the new territory.
The Missouri Compromise of 1820
When the residents of Missouri petitioned Congress for statehood in 1818, about 8,000 to 10,000 slaves lived in the territory. Southerners expected Missouri to join the Union as a slave state, but New York Congressman James Tallmadge introduced an amendment to the Missouri statehood bill that would have gradually ended slavery in the new state. The Tallmadge Amendment started a year of acrimonious debate in both houses of Congress.
Legislators finally reached a compromise in 1820, admitting Missouri as a slave state in exchange for admitting Maine as a free state. Wishing to avoid similar conflicts in the future, Congress also prohibited slavery in the rest of the Louisiana Territory, north of the southern border of Missouri (36°30′ north latitude). Although not a perfect solution in the minds of all parties (especially abolitionists), the Missouri Compromise curtailed the debate over the extension of slavery in the United States for over three decades.
The Mexican-American War
The slavery dispute re-emerged in 1846 when the U.S. started a war with Mexico that would greatly expand American territorial possessions in the West. Mexico and the U.S. had been at odds over Texas since 1836 when Texans won a war for independence from the Mexican government and established the Lone Star Republic.
Although the U.S., along with England and France, recognized the new republic, Mexico maintained hopes of reconquering the territory. In 1845, despite warnings from Mexico, Texans encouraged the U.S. Congress to annex the republic. When a dispute emerged over the border between Texas and Mexico, U.S. President James K. Polk sent 3,500 U.S. troops into the Lone Star Republic to prevent a Mexican invasion.
In November, Polk attempted to avoid hostilities by offering to purchase land that would settle the Texas border dispute, along with Mexican territories in the West coveted by the United States. Mexican officials dismissed Polk’s overtures. When war appeared to be imminent, Texans accepted an offer of annexation by the U.S. and became the 28th state on December 29, 1845.
The border dispute remained at an impasse over the winter, but on April 25, 1846, a Mexican cavalry force attacked a U.S. patrol that Polk had deployed into the contested territory between the Rio Grande and Nueces rivers. Polk requested a declaration of war and Congress obliged on May 13, 1846.
Congressman Wilmot’s Proviso
A little less than three months later, on August 6, Polk submitted a request to Congress to appropriate $2,000,000 to negotiate the end of the war by purchasing from Mexico the land he coveted. On August 8, 1846, Pennsylvania Congressman David Wilmot introduced an amendment to Polk’s request. Modeled from the language of the Northwest Ordinance, Wilmot’s Proviso stipulated the following:
Provided, That, as an express and fundamental condition to the acquisition of any territory from the Republic of Mexico by the United States, by virtue of any treaty which may be negotiated between them, and to the use by the Executive of the moneys herein appropriated, neither slavery nor involuntary servitude shall ever exist in any part of said territory, except for crime, whereof the party shall first be duly convicted.
House members voted to add Wilmot’s Proviso to Polk’s bill by a margin of 83–64 and then approved the entire bill by a vote of 85–80. On its face, Congressional support of Wilmot’s Proviso appeared to be a repudiation of the extension of slavery, but there were other political motivations behind the vote.
- Northern members of Polk’s party believed that his supporters had unfairly wrested the 1844 Democratic presidential nomination from New York’s Martin Van Buren. Polk further alienated Northern Democrats when he handed out patronage positions in New York to his Southern followers instead of to local Van Buren backers.
- Northerners, Whigs, and Democrats alike, believed that Polk’s administration was too pro-South. Southerners dominated his cabinet. Furthermore, Polk supported lower tariffs and opposed internal improvements at the expense of the North. Finally, Polk had settled for a compromise agreement in a dispute with Great Britain over the northern border of Oregon, while he was simultaneously promoting war with Mexico to expand Southern territories.
Whig leaders fixed on these grievances as an opportunity to level charges that the war with Mexico was being fought to extend slavery in the United States. Northern Democrats, well aware that those charges could prove damaging in the upcoming Congressional election of 1846, added their weight to the proviso.
After adoption by the House of Representatives, Polk’s appropriation request, including the Wilmot Proviso, moved on to the Senate, which was evenly split along sectional lines. Southerners successfully extended debate on the matter until the House adjourned for the season, thus ending the current session of Congress, and thereby killing the bill.
When Congress reconvened in 1847, Polk submitted another request for funds to end the war with Mexico. Northern Democrats led by Hannibal Hamlin and Preston King reintroduced a revised version of Wilmot’s Proviso that excluded slavery in “any territory on the continent of America which shall hereafter be acquired.” The House then passed Polk’s “Three Million Dollar Bill” along with the attached proviso by a vote of 115–106. In the Senate, where Southerners and Northerners held an equal number of votes, the anti-slavery measure was stripped from the legislation. Southerners then twisted enough arms in the House to get the Senate version passed.
The Treaty of Guadalupe Hidalgo
Controversy over the Wilmot Proviso erupted again in 1848 when the Mexican-American War ended and the administration sent the Treaty of Guadalupe Hidalgo to the Senate for ratification. Northerners attempted to interject the language of the proviso into the treaty, but Southerners again prevailed.
The acquisition of new territories from Mexico after the war amplified the debate over the extension of slavery, and the Wilmot Proviso evolved as a rallying cry for anti-slavery activists.
The enactment of the Compromise of 1850 averted bloodshed, but as Senator Salmon P. Chase guardedly observed, “the question of slavery in the territories has been avoided. It has not been settled.”
Because of its central role in the debate over slavery, the Wilmot Proviso may well be the most significant piece of legislation never passed by the U.S. Congress. Ominously, the measure was consistently defeated upon sectional, rather than party lines. By crystallizing the sectional differences that existed over slavery, the proviso transformed the political landscape in the U.S., beginning with the demise of the Whig Party and the emergence of the Free Soil Party, which eventually morphed into the Republican Party.
Free Soilers and Republicans adopted platforms advocating the congressional prohibition of slavery from all federal territories, giving the opponents of slavery a political party to rally around. Often characterized as “Black Republicans” by Southerners, those who supported the Wilmot Proviso emerged as clear antagonists for slavery advocates.
As the Whig Party collapsed, the Democratic Party did not escape unscathed. During the 1850s, Democrats gradually came to define themselves based upon their stance on the extension of slavery. Sectional differences hastened the division of the Democratic Party during the election of 1860, enabling Abraham Lincoln to ascend to the presidency with less than 40% of the popular vote, kindling the American Civil War.