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In 1845, Texas was finally allowed to become the 28th state of the United States after a contentious process of debate in the US Congress. Newly elected James K. Polk had developed an expansionist vision for the United States, where the Manifest Destiny was widely supported in Congress. California now also became the key prize, as that became seen as a wealthy place that would also give the United States great power by expanding to the Pacific Ocean. The United States also wanted to prevent Britain from gaining this territory. Soon, a border dispute arose between Mexico and the United States, mainly with the United States wanting the Rio Grande to be the border between the two countries. The United States sent a delegation to Mexico to negotiate the purchase of territory to California, but they were refused. President Mariano Paredes y Arrillaga then seized power in Mexico, where he did not recognize Texas' independence. Polk ordered General Zachary Taylor (himself a future president) to move into the disputed territory with Mexico, sparking the Mexican-American War. After some defeats, Santa Anna returned to power but this did little to change the fortunes of the war for Mexico. By 1848, Mexico had lost many of its key cities to the United States, forcing it to sign the Treaty of Guadalupe Hidalgo. This effectively created many of the western states of the United States and the border with Mexico to be defined near its present boundaries. The 1854 Gadsden Purchase, which was the purchase of land in southern Arizona and New Mexico, created what is effectively the current boundaries between Mexico and the United States. The creation of the new states and the war itself were extremely controversial in the United States, only surpassed by the larger slavery issue, where many had felt the United States acted belligerently by starting the war and forcing Mexico to cede land.<ref>For more on the Mexican-American War, see: Nardo, D. (1999). <i>The Mexican-American War</i>. San Diego, CA: Lucent Books. </ref>

The years after the war were still difficult for Mexico, as disease and Native American raids became a large problem. One of the provisions of the treaty required the United States to effectively deal with Commanche raids that become problematic for the United States and Mexico. This issue was not resolved for many decades and wars between Mexico, the Commanche, and the United States continued through the 1870s. What began to change, however, is northern Mexico began to orient economically more with the United States, as the region was somewhat disconnected with the power centers of Mexico to the south. Towns along the border soon began to boom economically.<ref>For more about the post-war years along the border in the 19th century, see: St. John, R. C. (2011). <i>Line in the sand: a history of the Western U.S.-Mexico border</i>. Princeton, NJ: Princeton Univ. Press. </ref>