 In June 1854 , McClellan was sent on a secret reconnaissance mission to Santo Domingo at the behest of Jefferson Davis . McClellan assessed local defensive capabilities for the secretary . ( The information was not used until 1870 , when President Ulysses S. Grant unsuccessfully attempted to annex the Dominican Republic . ) Davis was beginning to treat McClellan almost as a protégé , and his next assignment was to assess the logistical readiness of various railroads in the United States , once again with an eye toward planning for the transcontinental railroad . In March 1855 , McClellan was promoted to captain and assigned to the 1st U.S. Cavalry regiment . 
