 Complete the following phrase:  In March 1862, President Lincoln organized the Army of the Potomac into corps, and Heintzelman received the III Corps of the Army of the Potomac in the Peninsula Campaign. His corps played a prominent role in the Siege of Yorktown where Heintzelman and division commander Fitz John Porter were among the first to use the Union Army Balloon Corps. The corps bore the brunt of the fighting at Williamsburg and saw significant action at Fair Oaks, 