Theodore Lyman served on the staff of Major General Meade as aide-de-camp with a commission as lieutenant-colonel from Governor Andrew of Massachusetts. Lyman followed Meade until the end of the war from September 2, 1863, to April 20, 1865. During this time, he served as headquarters archivist. He also put his life on the line carrying flags of truce through hostile lines at Cold Harbor and Petersburg. His published letters and notebooks establish him as the preeminent recorder of events and personalities within the headquarters of Army of the Potomac.
Lyman was present at the Battle of Petersburg in June, 1864, wherein the Union forces suffered terrible losses. Lyman's June 25, 1864 diary entry speaks of these losses. In this entry, he writes: