 Complete the following phrase:  Walker volunteered for U.S. Army service in the MexicanAmerican War and was appointed first lieutet in the infantry on February 27, 1847. On April 9, 1847, he was assigned to the 1st U.S. Regiment of Voltigeurs and Foot Riflemen, a light infantry regiment. He was regimental adjutant between May 1, 1847 and June 15, 1847. Walker was appointed a brevet captain for his role in the 