The United States signed the Treaty of Guadalupe Hidalgo in 1848. This treaty marked the end of the Mexican-American War, which had begun in 1846. Under the terms of the treaty, Mexico ceded approximately 525,000 square miles of territory to the U.S., including present-day California, Nevada, Utah, Arizona, and parts of Colorado and New Mexico. The treaty significantly expanded U.S. territory and played a crucial role in the nation’s westward expansion. However, it also intensified debates over slavery's extension into new territories, contributing to the tensions that eventually led to the Civil War.