In any case, deterrence is not the magically foolproof force in international relations that its supporters assume. It depends on understanding and correctly signaling an adversary, and on that adversary behaving reasonably — none of which is guaranteed. We warned Saddam Hussein against invading Kuwait prior to the first Gulf War, but he misunderstood us. Back in 1941, Dean Acheson declared, "No rational Japanese could believe an attack on us could result in anything but disaster for his country." He was right, but from the (not entirely reasonable) Japanese perspective, the enormous risk seemed worth it. Even in the Cold War, deterrence nearly failed during the Cuban missile crisis.
Missile defense isn't a guaranteed success either, of course, but the system should be steadily upgraded so that its odds improve. The Republican House just cut the Bush administration's request for interceptors from 50 to 41 and axed funding for a third interceptor site to be based in Europe and geared toward the emerging Iranian threat. Instead, Republicans should be getting as many interceptors into the ground as soon as possible and also pursuing the promising airborne laser technology designed to zap enemy missiles when they are in their vulnerable boost phase (and far away from U.S. or allied territory).
The new world is upon us, fueled up and ready to go at a launch site in northeast North Korea. It only gets more frightening from here.