Rowley fought with the 45th Illinois volunteers at the Battle of Fort Donelson, in which General Grant's army captured the confederate fort, commanded by Brig. Gen. John B. Floyd, in February, 1862. After the battle Rowley was commissioned captain and transferred to Grant's staff as an aide-de-camp. Grant, having been promoted to major general, was permitted three aides and chose his friend and former neighbor from Galena as his third aide. After service at Fort Donelson, Rowley was moved to and was stationed at Savannah from March 4–11.