Arizona, south of the Gila River was legally bought from Mexico in the Gadsden Purchase on June 8, 1854. Tucson became a part of the United States of America, although the American military did not formally take over control until March 1856. In 1857 Tucson became a stage station on the San Antonio-San Diego Mail Line and in 1858 became 3rd division headquarters of the Butterfield Overland Mail until the line shut down in March 1861. The Overland Mail Corporation attempted to continue running, however following the Bascom Affair, devastating Apache attacks on the stations and coaches ended operations in August 1861.[citation needed]

Answer this question, if possible (if impossible, reply "unanswerable"): What year did the Overland Mail Corporation end operations?
1861