The annexation of Texas caused the Mexican ambassador to cut ties with the United States. The president of Mexico Jose Joaquin Herrera wanted to negotiate because he knew that Mexico did not have the resources for a war. Herrera also knew that the Mexican people would be outraged about the loss of Texas as it was a large territory. Herrera and Polk disagreed about the boundaries of Texas.
Mexico considered the Nueces River as the western border of Texas while Polk and Texans claimed that the Rio Grande was the border which made Texas much bigger. While the United States and Mexico continued to negotiate, Polk sent General Zachary Taylor to cross the Sabine River and to approach the Rio Grande in June 1845. Polk ordered that war was to commence if Mexicans crossed the Rio Grande. On April 25, 1846, a group of Mexicans crossed the border and 11 American soldiers were killed. War was declared on May 13, 1846.
Many opposed the war such as Abraham Lincoln, John Quincy Adams, and famously Henry David Thoreau. General Taylor was victorious in the first battles of the war due to superior American technology, but many American soldiers died due to heat and disease. The United States also set up a blockade around major Mexican ports. After a victory at Monterey, Polk attempted renegotiations but the new Mexican leader, Santa Anna, refused and in February 1847, the two armies met in the Battle of Buena Vista.
The battle ended in a tie which meant Santa Anna failed to meet his goal of destroying Taylor’s wounded army. The Treaty of Guadalupe Hidalgo was signed in 1848 and it ended the war. The treaty gave the United States the territories of Texas, New Mexico, and California. The United State became much larger with 90,000 new citizens who had been a resident of Mexico and more Indians. The Mexican-American War marked a turning point in the debate over slavery in the United States because of it reopening the Wilmot Proviso, the discussion about slavery in the debate over California’s statehood, and the Compromise of 1850.In 1846, Democratic Congressman David Wilmot proposed an amendment that prohibited slavery in any territory seized from Mexico.
The Wilmot Proviso was a creation of the anger of Northern Democrats at Polk’s administration which favored the South. The amendment passed the House but did not go through the Senate, where Southern Senators were able to block it. By 1850, the question of slavery created divisions between political parties.
The failed amendment increased the pressure to create a solution. The debate over slavery continued on in Congress because no solution was created.