Why Does Arizona Have That Diagonal Boundary Line?

Following the Treaty of Guadalupe Hidalgo ending the Mexican War in 1848 the boundary line between the United States and Mexico in what would become the future Arizona was the Gila River. One of the reasons the U.S. wanted to acquire Arizona and New Mexico was to build a southern wagon road and a transcontinental railroad to California. However, the best place to build a road and lay tracks was further south of the Gila so a railroad man from South Carolina named James Gadsden was sent to Mex