When Congress approved the annexation of the Republic of Texas, Mexico broke off diplomatic relations with the United States. Polk responded by ordering U.S. troops under the command of General Zachary Taylor into the new American state. He also sent his personal emissary, John Slidell, to Mexico City with a proposal to purchase New Mexico and California and fix the boundary of Texas at the Rio Grande. By March 1846, however, the Mexican government had been overthrown, the new Mexican president had reaffirmed Mexico's claims to all of Texas, Slidell's mission had failed, and Taylor's forces had advanced to the Rio Grande. Fighting began around Matamoros in April. When the news reached Washington a month later, Polk did not hesitate to send a war message to Congress, stating “Mexico has … shed American blood on American soil.” The fact that hostilities had broken out in still‐disputed territory was not considered particularly relevant. President Polk signed the declaration of war against Mexico on May 13, 1846.
The war in California and New Mexico. If Texas provided the spark for war, California provided the motive. The United States had long been interested in California, primarily because San Francisco had the finest natural harbor on the Pacific coast. In 1842, American naval forces, mistakenly believing that war had broken out between the United States and Mexico, landed at Monterey. Polk had confidential agents in place by 1844 to encourage American settlers in California to push for either annexation or independence under U.S. protection. On June 14, 1846, a small group of Americans in the Sacramento Valley ran a homemade flag up a pole and declared California an independent nation. The so‐called Bear Flag Revolt, which was supported by Captain John C. Frémont, was short‐lived. When the Mexican War actually did begin, Polk lost little time in sending the Pacific fleet under Commodore John Sloat to California, with orders to claim the province as occupied territory. Sloat landed at Monterey in early July and declared California part of the United States. Mexican resistance to the American takeover was over by January 1847. Virtually no fighting took place in New Mexico. Colonel Stephen Kearny arrived in Santa Fe in August 1846 with the “Army of the West,” a force of about seventeen hundred men, and simply proclaimed that New Mexico was new American territory. He established a temporary territorial government before moving on to California.
The war in Mexico. Polk had achieved his most important expansionist goals by the summer of 1846, but righting with Mexico continued for another two years. Taylor won important battles at Palo Alto and Monterrey in northern Mexico, making him a national hero. President Polk agreed to let Santa Anna, then in exile in Cuba, back into Mexico only if he promised to help negotiate a settlement. Santa Anna instead took command of the government and pledged continued resistance against the American invasion. Severely outnumbered, Santa Anna's forces were defeated by Taylor's troops at the Battle of Buena Vista (February 1847). The main theater of the war then shifted to the heart of Mexico. General Winfield Scott landed near Veracruz on March 29 and spent the spring and summer pressing the campaign toward Mexico City. The fall of the Mexican capital in September ended the war.
The Treaty of Guadalupe Hidalgo. Nicholas Trist, an official in the State Department, opened negotiations with Mexico in January 1848. The resulting Treaty of Guadalupe Hidalgo was ratified by Mexico in February and by the Senate in March. Under its terms, Mexico relinquished all claims to Texas north of the Rio Grande and ceded New Mexico and California to the United States. The lands of the Mexican Cession also encompassed Nevada, Utah, most of Arizona, and parts of Colorado and Wyoming. The United States agreed to pay $ 15 million for the new territory and an additional $3 million to assume the debt owed by Mexico to American citizens for past claims.