On August 27, 1862, at the age of nineteen, Albert Pope joined the Union Army attached to the 35th Massachusetts Volunteer Regiment, commissioned as a Second Lieutenant. The unit crossed the Potomac River on September 7, and just ten days later, fought at Antietam. The 35th Massachusetts confronted a Confederate crossfire and was stranded behind enemy lines with its ammunition exhausted before answering an order to retreat. Seventy-nine men from Pope's unit died that day. Pope survived a bout with cholera, and his unit served at the Battles of Fredericksburg, Vicksburg, and Knoxville. He mustered out as a Captain, though he received the honorary title of Brevet Lieutenant-Colonel for distinguished service. A Brevet title did not carry with it added authority or added pay.