The Treaty of Guadalupe Hidalgo, signed in 1848, marked a significant turning point in U.S.-Mexico relations. This treaty ended the Mexican-American War and resulted in Mexico ceding approximately half of its territory, including present-day California, Arizona, New Mexico, Nevada, and parts of Colorado and Utah, to the United States. The treaty not only expanded U.S. territory but also exacerbated tensions surrounding slavery in the newly acquired lands, contributing to the sectional conflicts leading up to the Civil War. Additionally, it formalized the U.S.-Mexico border, impacting border dynamics that continue to resonate today.