On May 13, 1846, the United States Congress formally declared war on Mexico, initiating the Mexican-American War. This conflict, lasting until 1848, significantly altered the territorial landscape of North America. 

The war stemmed from disputes over the annexation of Texas and the desire for territorial expansion under the doctrine of Manifest Destiny. President James K. Polk, a strong proponent of expansion, sought to acquire California and New Mexico. Tensions escalated when U.S. troops, under General Zachary Taylor, advanced into disputed territories, leading to skirmishes with Mexican forces.

The declaration of war was met with mixed reactions. Supporters viewed it as a fulfillment of America’s destiny, while critics, including then-Congressman Abraham Lincoln, questioned the legitimacy of the conflict.

The war concluded with the Treaty of Guadalupe Hidalgo in 1848, resulting in Mexico ceding a vast portion of its northern territories to the U.S., including present-day California, Arizona, New Mexico, and parts of several other states. This acquisition significantly expanded U.S. territory but also intensified debates over slavery’s expansion, contributing to the sectional tensions leading up to the Civil War.