Dana chose to follow the Union cause in the fall of 1861, and entered the Union Army as a colonel of the 1st Minnesota Volunteer Infantry Regiment on October 2. He was then given brigade command in Brig. Gen. Charles P. Stone's Division of the Army of the Potomac on October 20, an assignment lasting until March 23, 1862. During this time Dana and his men took part in the Union fiasco at the Battle of Ball's Bluff on October 21.