Robert E. Lee, as well as hundreds of officers in the U.S. Army, Navy, and Marine Corps from various seceding states, resigned his commission when his home state seceded. His resignation from the U.S. Army was followed by his acceptance to serve as a military advisor to CSA President Jefferson Davis and later a commission to lead the Army of Northern Virginia. In his resignation letter to the U.S. Secretary of War, he writes, "I have the honour to tender the resignation of my commission as Colonel of the 1st Reg. of Cavalry."
Photo courtesy the U.S. National Archives