In the ranger campaigns, Caldwell was "a very active Partisan", according to the commandant of Fort Niagara. Leading his troops into battle, he exhibited a ruthlessness that the Americans would never forget. On 3 July 1778, he was present at Battle of Wyoming in Pennsylvania. In September 1778 he led an attack on German Flatts in the Mohawk Valley of central New York, together with the Mohawk chief Joseph Brant's leading his Iroquois forces. They destroyed all of the buildings and grain in the area, and killed and captured much livestock, leaving the settlers' struggling through the winter.