An important turning point in the history of American expansion occurred during the Mexican-American War (1846–1848). Tensions between Texas and Mexico erupted into a full-scale war due to territorial issues and competing interpretations of the border between the two countries. The annexation of Texas by the United States and the ideology of “Manifest Destiny” only inflamed the need for expansion.

Some of the most notable engagements of the conflict took place in Palo Alto, Monterrey, and Buena Vista. California, Nevada, Utah, Arizona, New Mexico, and chunks of Colorado, Wyoming, Kansas, and Oklahoma were all awarded to the United States in the treaty that ended the war. This growth increased sectional tensions over slavery and helped pave the way for the onset of the American Civil War.

The war’s legacy is nuanced, encompassing imperial aspirations, arguments about the necessity of violence, and the deep effects on Mexican society and culture. Both the expansion of American territory and the complexity of foreign relations in the 19th century may be traced back to this event.