The event that led to the Gadsden Purchase in 1848 was the Treaty of Guadalupe Hidalgo, which ended the Mexican-American War. The treaty was signed on February 2, 1848, and as a result, the United States acquired significant territories from Mexico, including present-day California, Nevada, Utah, Arizona, New Mexico, and parts of Colorado and Wyoming. However, the exact negotiations and details of the Gadsden Purchase itself took place later, in 1853, when the United States acquired an additional strip of land from Mexico for the purpose of building a southern transcontinental railroad.
