On May 13, 1846, the United States Congress voted to support President James K. Polk's request to go to war with Mexico. The dispute between the United States and Mexico was over Texas.
Under the threat of war, the United States didn't annex Texas after it won its independence from Mexico in 1836. But in 1844, President John Tyler entered back into negotiations with the newly formed Republic of Texas, which ended with a Treaty of Annexation. At the time, the United States was in a delicate place regarding slave states and free states. The treaty was shot down by a majority of the Senate because it would throw off the slave state to free state ratio between North and South. Not only that, but it also risked war with Mexico, which had already ended its relationship with the United States. But, right before leaving office, and with the support of the President elect Polk, President Tyler was able to get a resolution passed on March 1, 1845. Texas got its statehood on December 29.
Mexico never followed through with their threats of war, but the relationship between Mexico and the United States was tense. There were ongoing border disputes, and in July of 1845, President Polk sent U.S. troops to the land that laid between the Neuces and Rio Grande rivers. That November the President send Joh Slidell, a diplomat, to Mexico. His mission was to discuss boundary adjustments and to make an offer to purchase the land that was California and New Mexico. After his mission failed, General Zachary Taylor lead the U.S. Army to the Rio Grande. The Rio Grande river was the river that Texas said was it's most southern boundary.
Mexico on the other hand, believed that the boundary was the Nueces River. Because General Taylor was moving troops to the Rio Grande, Mexico saw it as an act of aggression from the United States. In April of 1846 Mexico sent their own troops across the Rio Grande. President Polk, in response, saw the Mexican army's advance to be an invasion of U.S. land. On May 11, 1846, President Polk asked Congress to declare war on Mexico. It only took two days for Congress to do.
There was about two years of fighting before peace was finally established. The Treaty of Guadalupe Hidalgo was signed on February 2, 1848. Per the treaty, the Rio Grande river was made the southern boundary of Texas. Not only that, but California and New Mexico were to be added to the Union. In return, the United States paid Mexico $15 million and agreed to settle all claims of U.S. citizens against Mexico.
