Mexican–American War: the Battle of Chapultepec begins.

The **Mexican–American War (1846–1848)** was a major conflict between the United States and Mexico that dramatically changed the borders of North America. The war grew out of tensions following the American annexation of Texas in 1845. Mexico had never accepted Texas’s independence and considered its annexation by the United States an act of aggression. There was also disagreement over the southern boundary of Texas: the United States claimed the Rio Grande, while Mexico argued that the border was farther north at the Nueces River. After fighting broke out between American and Mexican troops in the disputed territory in April 1846, the United States declared war on Mexico the following month.

The year **1847 was the decisive period of the war**. American forces under General Zachary Taylor had already won important victories in northern Mexico, including battles at Palo Alto, Resaca de la Palma, and Monterrey. In February 1847, Taylor defeated a much larger Mexican army commanded by General Antonio López de Santa Anna at the **Battle of Buena Vista**. Although heavily outnumbered, Taylor’s troops held their position and forced Santa Anna to withdraw. The victory strengthened American control over northern Mexico and made Taylor a national hero, helping him win the U.S. presidency in 1848.

Meanwhile, the United States launched a second major campaign aimed directly at **Mexico City**. General Winfield Scott conducted an amphibious landing near Veracruz in March 1847, beginning one of the most ambitious American military operations of the era. After capturing Veracruz, Scott’s army advanced inland, defeating Mexican forces at Cerro Gordo, Contreras, Churubusco, and Molino del Rey. In September, American troops stormed **Chapultepec Castle**, which guarded the approaches to the Mexican capital. Mexico City fell to Scott’s army on September 14, 1847, effectively ending major organized resistance, although negotiations to formally conclude the war continued for several months.

The conflict formally ended with the **Treaty of Guadalupe Hidalgo**, signed on February 2, 1848. Mexico recognized the Rio Grande as the border of Texas and ceded an enormous territory to the United States, including present-day California, Nevada, Utah, and most of Arizona and New Mexico, as well as portions of other western states. In return, the United States paid Mexico $15 million and agreed to settle certain claims made by American citizens. The war greatly expanded U.S. territory and advanced the idea of **Manifest Destiny**, but it also intensified the bitter national debate over whether slavery should be permitted in the newly acquired lands. Those arguments deepened divisions between North and South and helped create political tensions that eventually contributed to the American Civil War.