In 1839 Haskin graduated from the United States Military Academy at West Point as 10th out of a class of 31 and was commissioned as 2nd lieutenant in the 1st U.S. Artillery. At the outbreak of the Mexican-American War he was a 1st lieutenant and received brevet promotions to captain for Cerro Gordo and major for Chapultepec. He was severely wounded during the battle Chapultepec which resulted in the loss of his left arm.