In 1846, the United States declared war on Mexico, marking a significant event that led to the Mexican-American War. This conflict emerged from territorial disputes, primarily over Texas, which had recently joined the U.S. The war fundamentally reshaped the boundaries of the nation, resulting in the U.S. acquiring present-day California, Nevada, Utah, Arizona, and parts of Colorado and New Mexico through the Treaty of Guadalupe Hidalgo in 1848. This expansion intensified debates over slavery in the newly acquired territories and set the stage for future sectional tensions leading up to the Civil War.