 Complete the following phrase:  In April 1862, Polk commanded the First Corps of Albert Sidney Johnstons Army of Mississippi at the Battle of Shiloh and continued in that role for much of the rest of the year under Beauregard, who had assumed command following the death of A. S. Johnston on the first day at Shiloh, and then under Gen. Braxton Bragg. At various times his command was considered a corps and at other times the Right Wing of the army. In the fall, during the 