This relief from the narrowly focused, obsessive fears that have dominated U.S. security policy for several decades is certainly a constructive development, but unfortunately it is not comprehensively valid. For strategic forces in particular, some subtle interaction between human judgement and physical capability remain potentially dangerous, presenting a security problem that will not be resolved simply by completing the projected agenda of national weapons development and international arms control agreements. The problem arises from conceivable combinations of events that are undoubtedly improbable but unprecedentedly catastrophic should any of them ever occur. The standards of safety that have evolved for improbable disasters of much smaller magnitude—nuclear reactor meltdown, for example—have been applied to only peacetime operations and have not been extended to the circumstances of advanced crisis or to the actual implications of combat operations. To appreciate the implications of this limitation and the dangers that emerge from it requires a substantial revision of standard perspectives on strategic security.