Many presidents, dictators, etc. came and went during a long period of instability which lasted most of the 19th century. One of the dominant figures of the second quarter of that century was the dictator Antonio López de Santa Anna.
Santa Anna was Mexico's leader during the conflict with Texas, which declared itself independent from Mexico in 1836 and ensured that independence by defeating the Mexican army and Santa Anna. Santa Anna was in and out of power again during the U.S.-Mexican War (1846-48). After accepting Texas's application for statehood in 1846, the US government sent troops to Texas in order to secure the territory, subsequently ignoring Mexico's demands for US withdrawal. Mexico saw this as a US intervention in their internal affairs by supporting a rebel province.
Disagreements about boundaries made the conflict inevitable. Mexican troops then attacked and killed several American soldiers and captured a small American detachment between the Rio Grande (which the Republic of Texas, and subsequently the U.S., claimed as the southern border) and the Nueces River (which had been considered the historic southern border of the Mexican department of Tejas). As a result, President James K. Polk requested a declaration of war, and the US Congress voted in favor on May 13, 1846. Mexico formally declared war on 23 May. This resulted in the Mexican–American War, which lasted from 1846 to 1848. Mexico was defeated by United States forces, which occupied Mexico City and many other parts of the country. The war was terminated with the Treaty of Guadalupe Hildalgo which stipulated that, as a condition for peace, Mexico was obligated to sell the mostly vacant northern territories to the United States for US$15 million. Over the next few decades, Americans settled these territories and petitioned for statehood, forming the states of California, Wyoming, Idaho, Oklahoma, Nevada, Utah, Arizona, New Mexico, and Colorado. Baja California was not included in the U.S. purchases or in the Treaty of Guadalupe Hildalgo despite being occupied by U.S. troops at the end of the war.
The primary reason for Mexico's defeat was its problematic internal situation, which led to a lack of unity and organization for a successful defense. One of the very few commemorated groups of Mexicans in the U.S. invasion of 1847 was a group of very young Military College cadets (now considered by some as Mexican national heroes). These cadets fought to the death defending their college against a detachment of American soldiers during the Battle of Chapultepec (September 13, 1847). Another group of combatants revered by Mexicans was the Batallón de San Patricio, a unit composed of hundreds of mostly Irish-born American deserters who fought under Mexican command until their overwhelming defeat at the Battle of Churubusco (August 20, 1847). Most of the "San Patricios" were killed and a few captured. Many of the captured men were court-martialled by the U.S. Army as deserters and traitors, and were subsequently executed at Chapultepec.
In 1853, mostly vacant desert territory containing parts of present-day Arizona and New Mexico were sold to the United States in the Gadsden Purchase. This land was sold by President Santa Anna in order to gain personal profit and to pay off his army. The Americans had not realized when they were negotiating the Treaty of Hidalgo (when they accepted the Gila River as the southern U.S. boundary) that a much easier railroad route to California lay slightly south of the Gila River. The Southern Pacific Railroad, the second transcontinental railroad to California, was built through this purchased land in 1881. As a bonus, the city of Tucson (Arizona) and its few hundred inhabitants was added to the United States territory of New Mexico.
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