This is a photo of the actual letter of resignation, that Robert E. Lee sent to the Secretary of War, Simon Cameron. Lee had to first resign his commission as a Colonel in the U.S. Army, before he could take command of the Confederate Army. Dated April 20, 1861.
To See Full Sized, click on Photo, then click on photo again on the next page.
Photo Courtesy of the National Archives, Record group 94, ARC Identifier 300372