"The very concepts of what constitutes offense versus defense and what is contained within either is a new and nascent debate in cyber," wrote Rod Beckstrom of the Homeland Security Department's National Cybersecurity Center, in an email. Historic concepts that have emerged include preemptive war strategy or when an attempt is made to combat a perceived inevitable threat before the threat becomes a reality and, the "no first use pledge," or when a state with nuclear weapons promises not to use the weapons first. Another approach, known as mutually assured destruction, "reflects the idea that one's population could best be protected by leaving it vulnerable so long as the other side faced comparable vulnerabilities," NuclearFiles.org states. The U.S. has waged past wars against threats that are not easily defined and geographically diverse such as terrorism and drugs.