The 91st joined a newly formed division on the 12th of September. Because other regiments were not properly equipped, they did not begin marching until the 14th. They were initially assigned to guard Frederick, Maryland, but on the 17th were ordered to join the Army at Antietam. They marched all night, arriving at dawn on the 18th. But McClellan decided not to attack on the 18th, and the Confederates left that night. They stayed for several days, during which William Reiff had a "ghastly adventure", which he later described in a letter to the National Tribune.
- Organization of the Army of the Potomac (relevant to the 91st PA)
- Humphreys' statement (an especially useful explanation by the division commander)
- Bates, p.186
- William C Reiff. 'His worst scare'. National Tribune 22 August 1895, page 3, column 2.
- 'Telegraphic news', Richland County Observer (Richland Center, Wisconsin), 10 October 1862, page 3
- court-martial record, RG 153, John Groft [note e.g. Robert Chism's testimony that the march was "very tiresome"]
- 'Lincoln's visit to Antietam', by William H Groninger (of the 126th Pennsylvania, which was then in the same brigade as the 91st PA) (reprinted from the Groninger Family Record)
- letter listing battles, Sellers to Bennett, 24 September 1864
- letter listing battles, Sellers to Bennett, 10 November 1864