The major historical event that involved the U.S. in a conflict with Mexico in the 1840s is the Mexican-American War, which took place from 1846 to 1848. The conflict arose primarily over territorial disputes after the U.S. annexation of Texas, which Mexico still considered its territory. The war resulted in significant military engagements and concluded with the Treaty of Guadalupe Hidalgo, through which Mexico ceded substantial lands to the U.S., including present-day California, Arizona, New Mexico, Nevada, and parts of Colorado and Utah. This expansion fulfilled the U.S. concept of Manifest Destiny but also intensified debates over slavery in the new territories.