The annexation of Texas by the United States provoked the Mexican ambassador to dissolve the connections between the two nations.
José Joaquín Herrera, president of Mexico, wanted to negotiate because he knew his country did not have the resources to wage war. During negotiations, the two presidents disagreed on the boundaries of Texas. Mexico believed Texas ended at the Nueces River while Polk claimed that the Rio Grande was the western border of Texas, which made Texas significantly larger. While discussions continued, Polk sent General Zachary Taylor to approach the Rio Grande. Polk declared that war would commence once Mexicans crossed the border. Finally, on April 25, 1846, several Mexicans crossed into America, engaged in battle with Taylor which lead to the death of 11 American soldiers. War against Mexico was declared on May 13, 1846. The United States went on to be victorious in battles due to superior American technology.
Additionally, the U.S. set up blockades around major Mexican ports to block Mexico from receiving supplies.
Despite the successes, many American soldiers were unaccustomed to the different environment and died from heat and disease. After capturing Monterey, Polk attempted renegotiations with Herrera, but was met with contempt, and in February 1847, the two armies met at the Battle of Buena Vista. The battle ended with no victor which meant Herrera failed to meet his goal of destroying Taylor’s wounded army. Eventually, the Treaty of Guadalupe Hidalgo was signed in 1848 and ended the war. It gave the United States the territories of California, Texas, and New Mexico.
The Mexican-American War reintroduced the question of the expansion of slavery with the events of the Wilmot Proviso, the debate over California’s statehood, and the Compromise of 1850. In 1846, Democratic Congressman David Wilmot proposed an amendment that would outlaw slavery within the newly acquired territories. The Wilmot Proviso was created due to the anger of Northern Democrats who believed Polk’s administration favored Southern states. The amendment passed the House but did not go through the Senate, where southern Senators were able to block it. The proviso reopened the debate over slavery. By 1850, debates over the expansion of slavery into the new United States territories continued to create divisions between political parties. The disappointment of the Wilmot Proviso increased the urgency to make arrangements concerning slavery in land acquired from the war while considerations proceeded inside Congress.
In 1849, California created a state constitution and applied for statehood. Admitting California as a free state would have balanced out the Senate; admitting Texas had tipped the balance towards slave states. Southern senators believed that more free states would follow California so they delayed any decisions. Northern abolitionism had been growing since the 1840s, therefore, northern states advocated for California’s admittance. Sectional divides were prominent during the discussion of California. Radical abolitionists believed that complete abolishment of slavery needed to be immediate. Moderate abolitionists were willing to let it stay where it already existed, but opposed expanding slavery into new territories.
Politicians continued to dispute over their differences of opinion.In 1849, President Zachary Taylor proposed implementing California as a free state, however, his proposal left New Mexico undetermined. Southern states threatened secession if two free states were admitted. Henry Clay suggested several proposals in hopes of avoiding a crisis. The first proposal suggested California be admitted without slavery while New Mexico was systemized under a territorial government with no policy on slavery.
New Mexico would have no Senatorial vote. The second compromise would settle the Texas-New Mexico border by giving New Mexico more land while Texas got $10 million to pay off its debts. The third proposal ordered the end of the slave trade in Washington D.C. Clay’s fourth amendment proposed legislation that allowed slave owners to reclaim runaway slaves. A final compromise was created by a new generation of senators – the Compromise of 1850. In the compromise, California joined the United States as a free state while New Mexico and Utah were organized into territories with no votes in Congress. The slave trade was forbidden in the District of Columbia.
Texas got $10 million in exchange for a smaller border with New Mexico. The Fugitive Slave Act was passed to give the South the right to reclaim runaway slaves with the ability to demand federal and local Northern help.