Summary
Highlights
In 1830, newly independent Mexico was in chaos after a decade-long war with Spain. The country was vast but lacked funds and faced internal rebellions. The US, meanwhile, was expanding, having purchased the Louisiana Territory and eyeing more land due to Mexico's internal problems.
Texas gained independence from Mexico in 1836. The US initially hesitated to annex it due to opposition from northern interests regarding a new slave state, and Mexico warned against it. However, President Polk, who campaigned on annexing Texas and reoccupying Oregon, initiated annexation procedures. Polk also sought to acquire California, New Mexico, and the Southwest. His offer to buy these lands was rejected, leading him to instigate conflict by moving troops into a disputed border zone between the Rio Grande and Nueces River. Texas was annexed on December 29, 1845, inheriting the border dispute and escalating tensions into war.
On April 25, 1846, Mexican cavalry attacked US soldiers in the disputed zone, prompting President Polk to declare war on May 13. While Mexico didn't officially declare war, the situation saw a weak Mexico facing an expansionist US. General Ulysses S. Grant, who served in the war, believed the US deliberately provoked the conflict. Meanwhile, Antonio Lopez de Santa Anna, a controversial figure who served multiple presidential terms, secretly dealt with US representatives to sell contested territory while pretending to reclaim Texas for Mexico. He later went back on his word, declared himself president, and prepared to fight the US.
US forces achieved victories at Palo Alto and Resaca De La Palma. General Stephen Watts Kearny advanced into New Mexico, entering Santa Fe unopposed on August 15 and claiming the territory for the US. He established a civilian government and set out for California. In December 1846, parts of Kearny's army moved into Northwest Mexico. A rebellion, the Taos revolt, occurred in the territory but was quickly subdued. In California, the US Navy had orders to seize San Francisco Bay. Captain John C. Frémont, leading a topographical expedition, returned to California when war was imminent, joining forces with Commodore Robert F. Stockton. Stockton's army entered Los Angeles unopposed on August 13. Despite some Mexican resistance, including the Battle of San Pasqual where General Pico's forces fought Kearney's, the American forces eventually defeated the Mexican forces at the Battle of Rio San Gabriel, and the Treaty of Cahuenga marked the end of armed resistance in California.
The Mexican Army's defeats caused political turmoil. Zachary Taylor's 2,300 US troops crossed the Rio Grande, fighting a hard-fought Battle of Monterrey. Despite initial Mexican repulsion in urban warfare, Taylor secured an armistice. Under pressure, he broke it and occupied Saltillo. Santa Anna marched north with 20,000 men to fight Taylor at Buena Vista. Despite being outnumbered, Santa Anna's forces suffered desertions and were tired. The battle was fierce, with the Mexicans nearly breaking US lines but suffering heavily from US artillery. Santa Anna retreated, citing a rebellion in Mexico City, allowing the US to hold its position. President Polk then sent a second army under General Winfield Scott to Veracruz to invade the Mexican heartland. Scott performed the first major amphibious landing in US history on March 9, 1847, leading to the surrender of Veracruz. Scott marched towards Mexico City, defeating Santa Anna's entrenched forces at the Battle of Cerro Gordo. Despite guerrilla harassment, Scott advanced on Mexico City, winning battles at Contreras and Churubusco. Peace negotiations broke down, leading to more battles at Molino Del Rey and Chapultepec, and the storming of the city gates. Scott occupied Mexico City and became its military governor.
Desertion was a significant problem for the Mexican army, largely due to soldiers' lack of loyalty to their generals and poor conditions. Despite calls to annex all of Mexico, the US decided against it due to cultural differences and potential instability. The Treaty of Guadalupe Hidalgo gave the US control of Texas, Alta California, and Nuevo México. In return, Mexico received 15 million dollars, which was less than half of what the US had offered before the war. The sparsely populated areas contributed to the low cost. With Texas and the secession from Mexico, the US gained over 900,000 square miles of land. The Gadsden Purchase later fixed the current border between Mexico and the USA.