The Mexican War lasted from 1846 to 1848, and was controversial largely because of its role in expanding slavery. More than a decade before the Civil War, the North and the South disagreed over the Mexican-American War, which the North saw as an opportunity for the South to expand "slave power." The cessation of the war led to a shaky compromise between the North and South that unraveled by 1860.

Slavery in Texas

In 1836, a piece of territory in Northern Mexico known as Texas rebelled and created its own "Lone Star Republic," the Republic of Texas. The region, though part of Mexico, had been increasingly occupied by white American settlers from the South before 1836. In addition, the region was a safe haven for escaping African-American slaves, because slavery was prohibited in Mexico. When Texas created its own republic, about 5,000 African-Americans lived in the territory. Texas, however, decided to allow slavery, and so these African-Americans lost all rights. Meanwhile, many in the United States, particularly in the South, were interested in admitting Texas as a state in the Union, which many Texans desired. This was initially rejected on the grounds that it would offset the balance of slave and free states, and that Mexico would retaliate with war. By 1845, however, Congress allowed Texas to the join the Union, which ignited conflict with Mexico.

Opposition to the Slave Power

Both the annexation of Texas and war with Mexico, however, were controversial, largely because of the issue of slavery. A dispute over Texas' southern border, however, prompted war, despite a minority of objections from Congress. Northern representatives opposed the war, believing it was a ploy to expand slave states in the Union and expand the institution westward. Famous opponents include Henry David Thoreau, who refused to pay his taxes in opposition and was imprisoned. A young Congressman from Illinois named Abraham Lincoln also voted against the war. In general, the war was waged largely by Southerners, who contributed the majority of the troops for the operation, despite having a smaller population than the North.

Wilmot Proviso

In an attempt to soothe tensions between the North and South, Pennsylvania Representative David Wilmot proposed his "Wilmot Proviso" in 1846, as the Mexican War was still brewing. Wilmot, an opponent of the war, decided that a compromise could be reached. Southerners desirous of new territory from Mexico could have that territory, but Northerners' worries could be calmed because the Wilmot Proviso would ban slavery in any newly-acquired lands. The House, dominated by Northerners, passed the Proviso. The Senate, however, refused to comply, and the Proviso never became law. Irrespective of its failure, the Proviso peaked national interest in the "Slave Power," which Northerners thought was dominating the federal government. Wilmot's Proviso highlighted a belief of many Northerners: the Mexican-American War was being fought for Southern, not American, interests.

Treaty of Guadalupe Hidalgo

In 1848, the Mexican-American War came to an end with the Treaty of Guadalupe Hidalgo. As part of the treaty, the United States gained 525,000 square miles of new land, which increased the pre-war size of the country by more than 50 percent. This land included parts of modern-day Colorado, Arizona, New Mexico, Wyoming, California, Nevada and Utah. While these territories were not yet open for slavery, their slavery status was undetermined, which prompted a vitriolic debate over slavery's expansion into the region. Southerners, who provided the majority of the manpower for the war, felt they had won the new territory on their own and deserved to expand slavery there. Northerners feared that the region would offset the balance of power in the federal government. This debate remained unresolved until 1860, when the Civil War began in part over this disagreement.