Portions of California, Arizona, New Mexico, Nevada and Utah previously were part of Mexico. Texas, Colorado and Wyoming also contain land formerly belonging to Mexico.
In 1848, the United States and Mexico signed the Treaty of Guadalupe Hidalgo, which ended the Mexican-American War and conveyed 525,000 square miles to the United States. The war began in 1846 under President James Polk because of disagreement about the border between Texas and Mexico. At the time, the United States claimed the Rio Grande as the border, while Mexico insisted on using the Nueces River. American troops invaded Mexico through the city of Veracruz and advanced overland to Mexico City. After Mexico City was defeated, the treaty was signed, ending the war in favor of the United States.
The Gadsden Purchase in 1854 further adjusted the border when the United States acquired portions of modern-day Arizona and New Mexico. President Franklin Pierce sent James Gadsden, minister to Mexico, to negotiate disputed land in the Mesilla Valley. Gadsden was authorized to spend up to $50 million, depending on the area of land negotiated. He met with Mexican President Antonio de Santa Anna, and they agreed on a sale of 45,000 square miles for $15 million, creating the present-day border.