The two nations involved in the Mexican-American War (1846-1848) were the United States and Mexico. The conflict was rooted in territorial disputes following the annexation of Texas by the U.S. in 1845, which Mexico still considered part of its territory. Tensions escalated into armed conflict, resulting in a series of decisive U.S. victories. The war concluded with the Treaty of Guadalupe Hidalgo, through which Mexico ceded a significant portion of its northern territories to the U.S., including present-day California, Arizona, New Mexico, and parts of Colorado and Nevada, fundamentally reshaping the boundaries of North America.