At that time, only about 75,000 Mexican citizens lived north of the Rio Grande. As a result, U.S. forces led by Col. Stephen W. Kearny and Commodore Robert F. Stockton were able to conquer those lands with minimal resistance. Taylor likewise had little trouble advancing, and he captured Monterrey in September.
With the losses adding up, Mexico turned to old standby General Antonio López de Santa Anna, the charismatic strongman who had been living in exile in Cuba. Santa Anna convinced Polk that, if allowed to return to Mexico, he would end the war on terms favorable to the United States. But when he arrived, he immediately double-crossed Polk by taking control of the Mexican army and leading it into battle. At the Battle of Buena Vista in February 1847, Santa Anna suffered heavy casualties and was forced to withdraw. Despite the loss, he assumed the Mexican presidency the following month.
Meanwhile, U.S. troops led by Gen. Winfield Scott landed in Veracruz and took over the city. They then began marching toward Mexico City, essentially following the same route that Hernán Cortés followed when he invaded the Aztec empire. The Mexicans resisted at Cerro Gordo and elsewhere, but were bested each time. In September 1847, Scott successfully laid siege to Mexico City’s Chapultepec Castle. During that clash, a group of military school cadets–the so-called niños héroes–purportedly committed suicide rather than surrender.