How could the Mexican-American War be considered a cause of the Civil War?
The issue of slavery was certainly contentious between the North and South before the Mexican-American War, but it had not reached a boiling point. For the most part, the free states and slave states were established and there seemed to be calm between the sections. With the victory over Mexico, the United States had a large tract of land to be settled. With this settlement came the issue of slavery and whether these new territories would institute a slave system. The 1848 Treaty of Guadalupe Hidalgo granted the United States the territories of Texas, New Mexico, and California. Since the United States desired a continuous nation that expanded from the Atlantic to the Pacific, the future of these territories needed to be determined. This would cause a great deal of stress between the northern and southern states.
The Missouri Compromise (1820) placed an arbitrary line on the map at 36 degrees, 30 minutes latitude as the boundary for slavery. Areas to the north of the line would be free territories while areas to the south would be granted slavery. This line seemed ineffective in the debate about new lands and territories. There were two competing arguments about slavery in these territories moving forward. John Calhoun, a Congressman from South Carolina, was the leader of the Southern cause. He believed that since slaves were property, owners should be able to take them wherever they pleased, including the new territories in the West. The South also feared that adding free states would jeopardize slavery as an institution as they would be further outnumbered in Congress.
The northern states opposed slavery in new territories on moral grounds and felt slavery would discourage poor laborers from moving west. Congress attempted to resolve these difference with the Compromise of 1850. Certain parts of the compromise caused tensions to escalate. First, California was to be a free state, which angered the supporters of slavery. The southern states were also disgusted that slavery was abolished in the District of Columbia. The territories of Utah and New Mexico would be allowed to vote on whether they wanted slavery or not. This angered the abolitionists in the North. The North was also upset about the Fugitive Slave Law that forced them to return escaped slaves to their owners. The Compromise of 1850 did very little to calm sectional differences and may have actually made matters worse.