The road to conflict for the United States and Mexico started a decade before the formal outbreak of war. In 1836, Texas won its independence from Mexico. Although they appealed to the United States for annexation, some in the US government balked at Texas’ inclusion as it would tip the balance between slave and free states. In addition, Mexico threatened war if the US moved to annex the Republic.
This changed when James K. Polk, a dedicated expansionist, was elected President in the election of 1844. Polk annexed Texas and offered to buy the territory that is now the Southwestern United States. Mexico refused. In response, Polk ordered troops south of the Nueces River, which was recognized as part of the Mexican state of Coahuila. On April 25, 1846, the Mexican cavalry attacked the US soldiers, who were under the command of Zachary Taylor. Several skirmishes followed. On May 13, Congress declared war; the United States was involved in its first war fought mainly on foreign soil.
Although Mexico valued the lands north of the Rio Grande River, they were sparsely populated. The US Army easily overran the area while Taylor and his men pushed into the Mexican heartland. Desperate, the Mexican government recalled the disgraced General Antonio Lopez de Santa Anna from exile in Cuba. Santa Anna had been in touch with Polk and promised the President to end the war on favorable terms to the US. Somewhat predictably, Santa Anna went back on his word as soon as he was on Mexican soil.
Installed as head of the Mexican Army, Santa Anna also assumed the Mexican presidency in March 1847. However, the Mexican forces were being pushed back. General Winfield Scott took Veracruz, the most important port city in Mexico, and advanced toward Mexico City. Following the path of Hernan Cortes three centuries before, Scott marched from the sea to the capital city. It fell in September 1847.
With the US Army on the streets of the capital, the war was over. Santa Anna resigned, forcing a new government to form and to negotiate the terms of a peace treaty. On February 2, 1848, the parties signed the Treaty of Peace, Friendship, Limits, and Settlement, better known as the Treaty of Guadalupe Hidalgo. The Rio Grande, not the Nueces River, marked the new boundary between the countries and it was decided based on scrutiny of the Disturnell map. Mexico finally had to recognize the loss of Texas and agreed to sell a huge swath of territory—the modern states of California, New Mexico, Arizona, and parts of Nevada, Utah, Wyoming, and Colorado—for a paltry 15 million dollars.