Afterwards, the Army turned its major attention back to the threat of a mid or high intensity conflict in Europe, and doubts reemerged about the value of helicopters in that sort of arena. Some military leaders believed that the helicopter could not survive and perform an essential role in a heavy combat environment. To gain general acceptance and ensure further success, Army Aviation continued to develop new doctrine, tactics, aircraft, equipment, and organizational structure. New or radically modified aircraft were adopted from the late 1970s into the mid-1980s. These included the Sikorsky UH-60 Black Hawk, Boeing AH-64 Apache, D-model of the CH-47 Chinook, and OH-58D version of the Kiowa.