1846 to 1848 - How the Mexican-American War Affected Slavery

The Mexican-American War took place between 1846 and 1848, yet its roots can be traced to 1821, when Mexico gained independence from Spain. At that time, Mexico encouraged Americans to settle in its sparsely populated northern territory on the conditions that settlers convert to Catholicism and renounce slavery. However, many of these settlers owned slaves and hoped eventually to secede from Mexico; in 1836, this is exactly what some settlers did to form the Republic of Texas. In the decade that followed, Texas remained an independent republic. At the same time, there was a growing sense among Americans that the United States had a “manifest destiny” to extend its territory to the Pacific Ocean, creating a nation “from sea to shining sea.” Many justified such expansion by arguing that it would bring freedom and enlightenment to the Native American and Catholic populations now living in those territories.

In 1845, the United States annexed the Republic of Texas and claimed that its southern border was the Rio Grande River. Mexico, however, claimed that the border was farther north at the Nueces River. This led to war in the spring of 1846, a conflict that lasted two years and cost the lives of nearly 14,000 American soldiers and an estimated 25,000 Mexicans. Hostilities ended in 1848 when, under the Treaty of Guadeloupe-Hidalgo, the United States paid Mexico $15 million for the acquisition of territory. This territory would ultimately yield four states (California, Nevada, Texas, Utah) and parts of four others (Arizona, Colorado, New Mexico, Wyoming).

The impact that new territory would have on the slavery question was anticipated during the war. In 1846, the U.S. House of Representatives passed the Wilmot Proviso, banning slavery in newly acquired territories; the Senate refused to take up this bill at that time or in subsequent sessions of Congress. Tensions rose in 1849 when California, which former President John Tyler had called the “crown jewel” of Mexican territories, formed a free-state government with a state constitution that outlawed slavery. This upset the balance of free and slave states that had been established in the Missouri Compromise of 1820 and maintained over the next three decades. Demands for another “Great Compromise” followed, with prominent northern and southern politicians on each side of the issue. The stalemate was broken in 1850, after two of the major politicians who opposed such a compromise, Senator John C. Calhoun and President Zachary Taylor, died during the spring and early summer. Illinois Senator Stephen Douglas then devised a negotiating strategy that dealt with the proposed law section by section, and the Great Compromise was enacted in September 1850.

While the North succeeded in getting California accepted as a free state, the South won the allowance of slavery in the remaining new lands (then designated as the New Mexico and Utah Territories). The South also won enactment of the Fugitive Slave Law, which required federal marshals to assist in the hunting of escaped slaves. The law also allowed private citizens to be deputized in these efforts, with the threat of imprisonment for those who refused. The act also established a hearings procedure in which African Americans were not allowed to testify and that paid commissioners $10 for findings in the favor of slaveholders, but only $5 for findings against slaveholders.

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