With the end of the last U.S.-Russian nuclear arms-control treaty, the world is entering a period of potentially unchecked nuclear weapon build-ups ¡ª a shift also expected to impact Japan and a debate over revising its three nonnuclear principles.

Thursday¡¯s expiration of New START marks the first time since the early 1970s that there will be no legally binding limits on the strategic forces of the world¡¯s two largest nuclear powers without a replacement agreement being under negotiation.

¡°The treaty¡¯s lapse removes taboos from certain policy debates, including those surrounding Japan¡¯s three nonnuclear principles,¡° said Michael Bosack, founder of the Parley Policy Initiative.