The agreement negotiated between the U.S. and Mexico in 1848 is known as the Treaty of Guadalupe Hidalgo. This treaty ended the Mexican-American War and resulted in Mexico ceding about half of its territory to the United States, including present-day California, Arizona, New Mexico, Nevada, and parts of Utah, Colorado, and Wyoming. This significant land acquisition contributed to the expansion of the U.S. and intensified the debate over slavery in the newly acquired territories, setting the stage for future conflicts leading up to the Civil War.