Deterrence [against North Korea] has worked remarkably well for more than 60 years as both sides understand the consequences of taking military action. The question is whether those ground rules are changing now. I don't believe they are. Still, miscalculation is a concern and too much rhetoric and idle chatter from both sides about preemptive strikes could lead one side to seriously consider taking action. There are extraordinary inhibitions in the use of force, but that's not a guarantee.