Final answer:
Gadsden purchased land in the southern U.S. to be used for a transcontinental railway, finalizing the border with Mexico and promoting Manifest Destiny.
Step-by-step explanation:
Gadsden purchased southern desert land to be used for the transcontinental railway. The Gadsden Purchase involved the acquisition of land in the southern part of present-day Arizona and New Mexico from Mexico. The United States desired this territory to facilitate the construction of a southern transcontinental railroad route and finalize border issues with Mexico after the Mexican-American War. This acquisition was considered the last major territorial addition to the continental United States and was instrumental in furthering the concept of Manifest Destiny.