The protection of cyberspace, the information medium, has become a vital national interest because of its importance both to the economy and to military power. An attacker may tamper with networks to steal information for the money or to disrupt operations. Future wars are likely to be carried out, in part or perhaps entirely, in cyberspace. It might therefore seem obvious that maneuvering in cyberspace is like maneuvering in other media, but nothing would be more misleading. Cyberspace has its own laws; for instance, it is easy to hide identities and difficult to predict or even understand battle damage, and attacks deplete themselves quickly. Cyberwar is nothing so much as the manipulation of ambiguity. The author explores these topics in detail and uses the results to address such issues as the pros and cons of counterattack, the value of deterrence and vigilance, and other actions governments can take to protect themselves in the face of deliberate cyberattack. For more than 60 years, decisionmakers in the public and private sectors have turned to the RAND Corporation for objective analysis and effective solutions that address the challenges facing the nation and the world.