The Obama administration has placed nuclear disarmament at the top of its foreign-policy agenda. Other possible goals, such as modernizing U.S. nuclear forces for deterrence purposes, are now considered either transitory or subordinate to taking steps toward “nuclear zero.” In itself, banning nuclear weapons is not a new U.S. goal; Ronald Reagan also supported it. But this prioritization is unprecedented. In the past, Republican and Democratic administrations have maintained a balance between the parallel goals of modernizing U.S. nuclear programs for deterrence and pursuing nuclear-arms reductions when feasible. This balancing act can be seen in the Clinton administration’s policy of...