On April 25, 1846, after the U.S. cavalry ignored an order from the Mexican army to retreat to the Nueces River and instead advanced south to the Rio Grande, fighting broke out. Three weeks later, Congress declared war on Mexico.

Cause

Fighting continued for more than a year, and ended in September 1847. In February 1848, the two countries signed the Treaty of Guadalupe-Hidalgo. The treaty recognized Texas as a U.S. state, and ceded a large chunk of land. The Mexican Cession included land that would later become California, Nevada, and Utah, as well as portions of Arizona, New Mexico, Colorado, and Wyoming.

Results