The history of Tex-Mex

Following President James K. Polk's election, the United States Congress declared war on Mexico on May 13, 1846. The combat began with a border skirmish in the Rio Grande, which was followed by a string of American successes. When the dust settled, Mexico had lost practically all of what is now California, Utah, Nevada, Arizona, and New Mexico.

In 1836, Texas declared its independence from Mexico. Initially, the United States refused to admit it to the union, due to northern political reluctance to the inclusion of another slave state. Border attacks were also encouraged by the Mexican government, which warned that any effort at annexation would result in war. Following Polk's election in 1844, annexation proceedings were swiftly launched, with Polk primarily campaigning on the idea of "re-annexing" Texas and "re-occupying" the Oregon Territory. Polk was also interested in California, New Mexico, and the rest of what is now known as the United States Southwest. When Polk's offer to buy those territories from Mexico was turned down, he started a conflict by sending troops into a territory between the Rio Grande and the Nueces Rivers, which both countries had previously recognized as part of the Mexican state of Coahuila.

As US forces pushed across Mexican territory, they wanted to ensure that the areas they occupied were well-governed, especially since Polk's strategy relied on suppressing anti-American insurgencies in those areas.  General Taylor's attack south of Texas involved capturing Mexican land that Polk did not want the US to maintain, and the War Department gave Taylor precise instructions on how to administer it militarily. Polk, on the other hand, wanted to take and maintain New Mexico and California. When Kearny arrived in New Mexico, he used his discretion to declare not only that it was now U.S. territory, but also that its residents who were loyal to the US would be recognized as citizens (those who were disloyal could be tried and hanged). He then issued new law codes and appointed civilian officials such as the governor and supreme court justices. In California, Kearny and other commanders were a little more cautious in forming a military occupation administration. In his annual report to Congress in December 1846, Polk noted that, in accordance with international law, American forces had created temporary administrations in New Mexico and California pending the outcome of the war and the signing of a peace treaty.

The Treaty of Guadalupe Hidalgo, signed in 1848, ended a two-year conflict between the United States and Mexico. The treaty established the Rio Grande as the southern Texas boundary and gave the United States $15 million in exchange for Mexico's northern provinces (which now include California and substantial portions of New Mexico, Arizona, Nevada, Utah, and Colorado). The Senate signed into law the treaty in July of that year. Polk praised the benefits of the American constitutional system for waging war in his 1848 annual address to Congress, following the ratification of the Treaty of Guadalupe Hidalgo: The war with Mexico has thus fully developed the capacity of republican governments to prosecute successfully a just and necessary foreign war with all the vigor usually attributed to more arbitrary forms of government.”