While Japan has made nuclear disarmament a pillar of its diplomacy, in fact the existence of U.S. nuclear weapons has been critical to Japan's postwar peace and prosperity. The U.S. nuclear arsenal provided an extended deterrent — a nuclear umbrella — that sheltered this country and protected it from external threats.

Japan was (and is) not the only country to nestle under that shield: All U.S. allies enjoyed a similar status. The United States is the only country today that affords its allies this protection and that role complicates otherwise straightforward policy adjustments in the post-Cold War era.

Since taking office, U.S. President Barack Obama has tried to change the course of U.S. nuclear policy. His call to work toward the elimination of all nuclear weapons was not just campaign rhetoric. In his speech in Prague last year, Mr. Obama laid out his vision of a nuclear-free world. The new U.S.-Russia Strategic Arms Reduction Treaty, signed just weeks ago, is a step toward the realization of that vision. But a treaty is not a policy; it is a means to an end. Every U.S. administration conducts a Nuclear Posture Review (NPR), a document that outlines U.S. nuclear policy and strategy. The NPR provides guidance for U.S. nuclear planners, but it also explains how an administration thinks about nuclear threats, nuclear weapons and how it intends to deal with them.