The territory disputed by Spain and the United States leading to a war in 1846 was Texas. After gaining independence from Mexico in 1836, Texas was annexed by the United States in 1845, which escalated tensions with Mexico, leading to the Mexican-American War. The war was fueled by the desire of the U.S. to expand its territory, a concept known as Manifest Destiny. The conflict lasted from 1846 to 1848 and resulted in significant territorial gains for the U.S., including present-day California, Nevada, Utah, Arizona, and parts of Colorado and New Mexico, outlined in the Treaty of Guadalupe Hidalgo.