Texashad been an independent republic for nine years, having gained independence in 1835 after defeating the Mexican Army at the Battle of San Jacinto. It was recognized as a nation by France, Great Britain, and the United States, but the goal of many Texans was to become part of the United States.
Many Americans also felt that as part of the nation's? "Manifest Destiny" to stretch from the Atlantic to the Pacific, the United States should take steps to annex Texas, the Oregon Territory, California, and the rest of the southwest. In 1844, James K. Polk was elected president, and he began to realize this dream. Just before President John Tyler left office in 1845, Congress passed a joint resolution to annex Texas. Polk then sent an agent to California to support a revolution against Mexican control if one should break out. He also sent John Slidell to Mexico with an offer to buy New Mexico and California for $30 million, but Mexican officials refused to talk to him. In 1846, the boundarydispute with Great Britain over the Oregon Territory was finally settled. The northern border of the United States would be extended along the 49th parallel to the Pacific Ocean.
Trouble with Mexico erupted over a border dispute in Texas. The United States claimed that the border between Texas and Mexico should be the Rio Grande River, while the Mexicans claimed the border was farther north at the Nueces River. President Polk ordered General Zachary Taylor to take an army into the disputed region, and in April 1846, one of Taylor's patrols was attacked by Mexican troops. Claiming that? "American blood has been shed on American soil"? Polk asked Congress to declare war. Congress passed the declaration of war, although congressmen such as Abraham Lincoln and John C. Calhoun believed the war was just an excuse to seize land.
The Americans attacked on several fronts. Taylor's army advanced into the region south of the Rio Grande, winning victories at Palo Alto and Resaca de la Palma in May 1846. He captured Monterrey in September, and his victory over Santa Anna at Buena Vista in February 1847 ended fighting in northern Mexico. An army led by General Stephen Kearny captured Santa Fe and moved across the deserts and mountains into California. General Winfield Scott was sent to capture Mexico City. After victories at Vera Cruz and other cities along the way, Scott captured the capital city on September 14, 1847.
Congress approved the Treaty of Guadalupe Hidalgo to end the war on February 2, 1848. Mexico agreed to give up all claims to Texas, establish the Texas border at the Rio Grande, and cede California and the New Mexico territory to the United States for $15 million. This added over 500,000 square miles to the United States.