What Area Did the US Acquire During the Mexican War?

The Mexican War, instigated over a border dispute between the U.S. and Mexico, culminated with huge territorial gains for the United States. Known as the Mexican Cession, the area included more than 500,000 square miles and all or parts of modern-day California, Nevada, Utah, Wyoming, Colorado, Arizona and New Mexico. The U.S. purchased this land for $15 million, and settlers populated the area within a few decades.

The Mexican War began as a dispute over the boundaries of the United States' newly-acquired territory, Texas. For Mexico, Texas' boundary ended at the Nueces River, but Texans and Americans thought it ended farther south at the Rio Grande. After two years of war, and Mexico's defeat, this was the first piece of territory the country conceded to the United States. During the negotiations for the Treaty of Guadalupe-Hidalgo, Mexico agreed to allow the United States to take territory extending up the Rio Grande. Still today, this river creates a significant part of the border between the United States and Mexico.

Though the Mexican War was ostensibly fought over a tiny piece of Texan territory, the idea of "Manifest Destiny" motivated the country to want more territory. The grand prize was the west coast of North America, a part of Mexico known as California. Securing the region would fulfill the goal of a country that spanned from the Atlantic to the Pacific. Initially, the United States wanted both Baja and Alta California. The Mexicans, however, only consented to the transfer of Alta California, which included everything from the Port of San Diego northward towards and beyond San Francisco. Though sparsely populated at the war's end, in 1848 the discovery of gold in California brought the territory's population from 14,000 to more than 250,000 in just four years.

Americans were not satisfied with an isolated California discontinuous from the rest of the United States. As part of the Treaty of Guadalupe Hidalgo, the United States also acquired all the Mexican territory between California and Texas. This included occupied regions such as New Mexico and Arizona. In addition, all or parts of Nevada, Utah, Wyoming and Colorado were transferred to the United States. The precise border between the U.S. and Mexico in this region was not immediately determined in the treaty. Instead, a group of future surveyors were to survey and negotiate until an agreement on the exact border was reached.

For the United States, the Mexican Cession represented 55 percent of its prewar territory. This, however, was a smaller gain than some, including President James Polk, desired. Many Northern abolitionists opposed the annexation of Mexican territory, and the president and his supporters compromised their hopes in the interests of political unity. For Mexico, the loss of the Cession was devastating, as it represented a loss of nearly one-half of the nation's prewar territory. Valuable resources in the agricultural- and gold-rich California region helped propel the United States' economic development.