Today, sophisticated cyber capabilities reside almost exclusively in nation-states. Here, U.S. military power offers a strong deterrent against overtly destructive attacks. Although attribution in cyberspace can be difficult, the risk of discovery and response for a major nation is still too great to risk launching destructive attacks against the United States. We must nevertheless guard against the possibility that circumstances could change, and we will have to defend against a sophisticated adversary who is not deterred from launching a cyber attack.