Part the last of our series on interesting facets of the Mexican War concludes with the 1848 peace Treaty of Guadalupe Hidalgo, which gave the United States full ownership of Texas, with its western border at the Rio Grande, and the modern States of California, Nevada, Utah, New Mexico, almost all of Arizona, Colorado, and part of Oklahoma, Kansas, and Wyoming (the rest of Arizona and New Mexico would be acquired through the 1853 Gadsden Purchase). In return, Mexico received a little over $18 million in compensation and forgiveness of $3.25 million owed by Mexico to the U.S.
Immense as the territories ceded by Mexico were, there were a number of U.S. Senators who urged Congress to take advantage of Mexico’s internal political chaos and force it to also give up its states of Nuevo León, Coahuila, and Tamaulipas, in today’s northeastern Mexico. This would have extended the U.S. hundreds of miles beyond Texas’ current southern border. Partly because there was growing opposition to the war in the U.S. (Illinois Rep. Abraham Lincoln was opposed), and partly because the parts of Mexico that the U.S. had so long desired, particularly California, were already handed over, Congress declined to pursue the war any longer, and this plan was dropped.
The Mexican Cession was at once a great acquisition for the U.S. and the end of the U.S. as it had been. The new lands made the slavery debate impossible to resolve through political compromise. The 1820 Missouri Compromise would have allowed slavery in New Mexico, Arizona, and the southern half of California, but not in Colorado, Utah, Kansas, or Wyoming. But anti-slavery Americans were not about to let California, the greatest prize of them all, the one that held out the most promise to small farmers and free labor, become a slave state (since a state could not be half-free, half-slave, California ran the risk of becoming a full slave state). Pro-slavery Americans knew that New Mexico and Arizona were not lands that lent themselves to plantation farming, and determined more fiercely than ever to have California, and the farmland that would become Kansas, too.
Free-Soil, free-labor, anti-slavery, and abolitionist Americans said now was the time to contain slavery altogether—to see the new territories not in the context of the north-south line of the Missouri Compromise, but as The West, a new entity that was not bound by the north-south politics or agreements of the eastern states. Keep slavery out of The West, they said, and keep it contained in the southern states until slave states were so outnumbered by free states, and slavery such an anomaly in the country, that slavery itself would die out.
Pro-slavery Americans had been ready for this fight for years. The nation had expanded along the Missouri Compromise line for nearly 30 years, it was the law of the land, there was no reason to change it, and any anti-slavery agitation in The West would be illegal, and punishable by law.
The problems the Mexican Cession caused would have to be quickly hammered out in the Compromise of 1850, a five-part piece of legislation that tried to create true compromise between anti- and pro-slavery Americans, not along purely geographical lines, but more philosophically. Slavery was not banned in the West (1), but California would enter the Union as a free state, end of story (2). Each of the remaining western territories that wanted to become a state would decide on its own whether to come in as slave or free: popular sovereignty let the people in the territory vote on their status before applying for statehood (3). The Fugitive Slave Act was introduced, which allowed slaveholders to violate the personal liberty laws in free states (4), and slavery would remain a feature in the capital, Washington, DC (5).
This Compromise would be short-lived. As settlers poured into all regions of the Cession, the stakes became higher and higher on both sides of the slavery issue. Pro-slavery Americans needed numbers; they couldn’t allow slavery to be restricted to the existing southern states or their needs would never be met in Congress, where free-state Representatives and Senators would far outnumber slave. Anti-slavery Americans also needed numbers, to reduce slavery to a regional curiosity of a small number of states, rendered economically useless. The battles over how western states would come into the Union led to vote-rigging, where people from outside a territory would pour in when it came time to vote slave or free, making a mockery of the concept of popular sovereignty. The violence that ensued in these situations was made legendary in Bloody Kansas.
In short, the Mexican War was most important both for expanding the U.S. and for hastening the coming of the Civil War. Both events made the nation greater, one geographically, one morally. It was a dress-rehearsal for the Civil War in that so many men who fought together in the Mexican War fought against each other in the Civil War, including both Ulysses Grant and Robert E. Lee. And it nearly completed the U.S. conquest of the continent between Canada and what was left of Mexico (the last bits settled in the Gadsden Purchase). The discovery of gold in California the year after the war ended spurred not only Californian settlement but the western rush of pioneers that dominated American demographics until the end of the 19th century. It also left the United States as the undisputed great power of the western hemisphere—a great deal of impact for a war that is often skipped over as students of U.S. history move from the Revolutionary War directly to the Civil War.