The Mexican Cession was a significant event because it marked the end of the Mexican-American War and resulted in the large-scale territorial expansion of the United States. Under the terms of the Treaty of Guadalupe Hidalgo in 1848, Mexico ceded a vast amount of territory to the United States, including present-day California, Nevada, Utah, Arizona, and parts of New Mexico, Colorado, Wyoming, and Idaho. This acquisition of land not only greatly expanded the geographical size of the United States, but it also helped to fulfill the concept of Manifest Destiny, the belief that it was the destiny of the United States to expand westwards to the Pacific Ocean. The Mexican Cession also led to debates and conflicts over the issue of slavery in the newly acquired territories, which ultimately contributed to the outbreak of the American Civil War in 1861.
