Final answer:
James Gadsden was sent to Mexico to purchase land for the US in order to secure a path for the construction of a southern transcontinental railroad and fulfill the beliefs of Manifest Destiny.
Step-by-step explanation:
James Gadsden was sent to Mexico in 1853 to purchase a strip of land for the United States, a transaction known as the Gadsden Purchase. This was done primarily because of reason 1 and reason 3. The US government wanted to secure a path to facilitate the construction of a southern transcontinental railroad, which is a route largely free from high altitudes of the Rocky Mountains providing the easiest north to south route.
Manifest Destiny, the belief that the US was destined to expand across the continent, also played a significant role to an extent. The notions tied to King Cotton and popular sovereignty in this context tend to be more debatable.
Learn more about Gadsden Purchase