Did the Size of America Increase Dramatically After the Mexican-American War?

Many American political leaders in the 1840s believed that it was the destiny of the United States to build a nation that stretched from the Atlantic to the Pacific. However, at the time, Mexico claimed much of the territory between the western border of the United States and the Pacific Ocean. James Polk ran for president in 1844 largely on his platform that the United States should annex Oregon -- which belonged to Great Britain -- and Texas, which had declared independence but was still claimed by the Mexican government.

Under President Polk's leadership, the United States annexed Texas in 1845. This added a significant amount of territory to the United States, much of which Mexico still claimed. The Mexican government not only refused to officially recognize Texan independence, but it also placed the southern border of Texas at the Nueces River at more than 150 miles north of the Rio Grande, which Texas and the United States claimed as the border. President Polk sent a representative -- John Slidell -- to Mexico to purchase the disputed territory, California and New Mexico, but the Mexican president refused to see Slidell.

Border skirmishes were common between Mexicans and Texans and Americans in the 1840s. One such skirmish that occurred north of the Rio Grande resulted in a handful of American casualties and served as the pretext for war. The United States won a series of battles, eventually laying siege to Mexico City. With few legitimate options, Mexico surrendered. The peace treaty negotiated, the Treaty of Guadalupe Hidalgo, set the U.S.-Mexico border at the Rio Grande, and ceded land that comprises all or most of the current U.S. states of California, New Mexico, Arizona, Nevada and Utah and the western portion of Colorado. In return, the United States paid Mexico $15 million, agreed to settle debts between Mexico and U.S. citizens and promised to protect the Mexican border from attacks by Native Americans.

The United States purchased an additional 29,670 square miles of land from Mexico for $10 million in 1854. Some of the land, comprising the southern portions of present-day Arizona and New Mexico, was disputed territory -- claimed by both countries. The U.S. wanted the land because it was ideal for building a southern transcontinental railroad. Mexican President Antonio Lopez de Santa Anna sold the land in part because he needed money to fund his army to put down rebellions in Mexico. The purchase also resolved most lingering border disputes between the two nations.