 Complete the following phrase:  In 1838, he commanded the cavalry instructional establishment at Carlisle Barracks in Pennsylvania. He was assigned to Ft. Atkinson, Iowa Territory, from 1842 until 1845. He was the forts commander during most of that period. He was promoted to major of the 2nd Dragoons on June 30, 1846. During the MexicanAmerican War, Sumner was brevetted for bravery at the Battle of Cerro Gordo to lieutet colonel. It was here that he gained the nickname Bull Head because of a story about a musket ball that bounced off his head during the battle. At the 