The Mexican War, also known as the Mexican-American War, occurred between 1846 and 1848. It was primarily fought over territorial disputes following the annexation of Texas by the United States. The conflict began when U.S. troops were sent to the disputed area between the Rio Grande and Nueces River, leading to clashes with Mexican forces. The war ended with the signing of the Treaty of Guadalupe Hidalgo in 1848, which resulted in the U.S. acquiring significant territories, including present-day California, Arizona, New Mexico, and parts of Colorado, Nevada, and Utah. This expansion intensified debates over slavery in the newly acquired lands.