The 1997 Quadrennial Defense Review (QDR) evaluated future U.S. strategy and force structure. It paid particular attention to asymmetric threats that attack vulnerabilities not appreciated by the United States and capitalize on limited U.S. preparations against such threats. These threats rely primarily on concepts of operation, such as an adversary using chemical and biological weapons to attrite and disrupt U.S. forces, that differ fundamentally from those employed by the United States. The threats also can employ new or different weapons. U.S. conventional superiority has forced adversaries to consider asymmetric threats. Asymmetric strategies are not new; much of military history and theory focuses on asymmetric challenges. But the United States often fails to appreciate these threats, and therefore does not adequately prepare for them. In modern warfare, the performance of U.S. forces could be substantially degraded if they are faced with threats for which they are inadequately prepared, including the needed doctrine, operational concepts, military R&D and acquisitions, force structure and force posture, and training. While U.S. doctrine as explained in Joint Vision 2010 calls for "full-dimensional protection" of U.S. forces, such protection is extremely expensive and does not presently exist.