 Although the short-range AIM-9 was thought to be an effective weapon, the use of the beyond visual range AIM-7 was constrained by Rules of Engagement which in most cases required visual confirmation of a target before firing – essentially defeating any advantage that the missile would have conferred to the American pilots.Both missiles had exhibited reliability problems in 1966 combat, exacerbated by maintenance problems caused by the tropical conditions of Southeast Asia, with the majority failing to ignite, fuse, or guide to the target.The F-4's primary adversary during this engagement was the Soviet-built Mikoyan-Gurevich MiG-21, NATO reporting name 'Fishbed', a small fighter designed as a short-range interceptor, a role that perfectly suited its use by the VPAF.