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The really difficult problem will be a new kind of proliferation involving the acquisition of chemical, biological and cyberweapons by subnational actors such as terrorist groups, cults or angry individuals. These weapons are easy to make, hard to track and hard to defend against. This means that even if the U.S. does spend tens of billions of dollars on a system to shoot down North Korean missiles, we will still have to deal with the equally pressing problem of stopping doomsday cults or future Unabombers armed with deadly viruses.
Just as internecine conflicts have risen while interstate wars have declined, so subnational terror may increase as traditional nuclear threats wane. Monitoring and deterring states are hardly easy, but they pale in difficulty compared with the task of monitoring and deterring obscure and fleeting groups and individuals. Not everyone in our future will have a bomb, of course. If countries really want to, though, a lot of them will still be able to lay their hands on the stuff of nightmares.
Gideon Rose is Olin Senior Fellow for National Security Studies at the Council on Foreign Relations and senior editor of Foreign Affairs