The Mexican War and the Wilmot Proviso

This entry is part 9 of 18 in the series The Roots of the Civil War
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David Wilmot-The Mexican War and the Wilmot Proviso The Wilmot Proviso was a failed attempt by the so-called Barnburner wing of the Democrat Party to ban slavery in any territory to be acquired from Mexico in the Mexican War or in the future.  The term barnburner was derived from the idea of someone who would burn down his own barn to get rid of a rat infestation, in this case slavery. Led by David Wilmot, a Democrat congressman from Pennsylvania, they were opposed to the further spread of slavery.

The fight to stop the spread of slavery was caused by the Mexican War that lasted from April 25, 1846 until February 2, 1848. This period in American history was driven by the spirit of Manifest Destiny. It was a widely held belief that it was America’s destiny to spread all of the way to the Pacific Ocean.

In 1836 the Texans had won their independence and founded the Republic of Texas but pressure began to build almost immediately for Texas to become a state. The annexation of Texas was contentious in the U.S. Congress, where Whigs were largely opposed. Finally, in 1845 Texas agreed to the offer of annexation by the U.S. Congress and became the 28th state on December 29, 1845. Texas entered the Union as a slave state.

The Mexican War began as a boundary dispute between the United States and Mexico. Simultaneously, John C. Fremont and a group of armed men appeared in California. He raised the American flag at a fort on Gavilan Peak but left California in March 1846. However, he returned to California and assisted the Bear Flag Revolt in Sonoma, where many American immigrants stated that they were playing “the Texas game” and declared California’s independence from Mexico.

Tensions between the two countries continued to heat up despite attempts by the American government to negotiate with their southern neighbor. Mexico was not inclined nor able to negotiate. In 1846 alone, the presidency changed hands four times, the war ministry six times, and the finance ministry sixteen times. Mexican public opinion and all political factions agreed that selling the territories to the United States would tarnish the national honor.

Talks broke down when a more nationalistic government came to power. President James K. Polk ordered General Zachary Taylor and his forces south to the Rio Grande, entering the territory that Mexicans disputed. Mexico laid claim to all the lands as far north as the Nueces River—about 150 mi  north of the Rio Grande.

The U.S. claimed that the border was the Rio Grande, citing the 1836 Treaties of Velasco. Mexico rejected the treaties and refused to negotiate; it claimed all of Texas. Taylor ignored Mexican demands to withdraw to the Nueces. He constructed a makeshift fort (later known as Fort Brown/Fort Texas) on the banks of the Rio Grande opposite the city of Matamoros.

On April 25, 1846, a 2,000-strong Mexican cavalry detachment attacked a 70-man U.S. patrol under the command of Captain Seth Thornton, which had been sent into the contested territory north of the Rio Grande and south of the Nueces River. In the Thornton Affair, the Mexican cavalry routed the patrol, killing 16 American soldiers.

The war lasted slightly more than a year and a half with the complete defeat of the Mexican forces. The Treaty of Guadalupe Hidalgo, signed on February 2, 1848 ended the war. The treaty gave the U.S. undisputed control of Texas, established the U.S.-Mexican border of the Rio Grande, and ceded to the United States the present-day states of California, Nevada, Utah, New Mexico, most of Arizona and Colorado, and parts of Texas, Oklahoma, Kansas, and Wyoming.

In return, Mexico received US $15,000,000 ($492,399,038 today) – less than half the amount the U.S. had attempted to offer Mexico for the land before the opening of hostilities and the U.S. agreed to assume $3.25 million ($87,687,500 today) in debts that the Mexican government owed to U.S. citizens.

Enter the anti-slavery forces who attempted to halt the spread of slavery by passing the Wilmot Proviso as a rider on a $2,000,000 appropriations bill intended for the final negotiations to resolve the Mexican–American War. Their first attempt was in August 1846, just three months into the war. It passed the House but failed in the Senate, where the South had greater representation.

The amendment was brief and to the point:

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.

It was reintroduced in February 1847 and again passed the House and failed in the Senate. In 1848, an attempt to make it part of the Treaty of Guadalupe Hidalgo also failed.

With the introduction of the Wilmot Proviso the issue of the spread of slavery moved from one of abstraction to one involving practical matters. t soon became clear to the South that this long postponed attack on slavery had finally occurred. Rather than simply discuss the politics of the issue, historian William Freehling noted, “Most Southerners raged primarily because David Wilmot’s holier-than-thou stance was so insulting.” 

Combined with other slavery related issues, the Wilmot Proviso led to the Compromise of 1850, which helped buy another shaky decade of peace. Radical secessionists were temporarily at bay as the Nashville Convention failed to endorse secession. Moderates rallied around the Compromise as the final solution to the sectional issues involving slavery and the territories.

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