Treaties are already in place to cap the number of nuclear warheads that the U.S. and Russia can have ready to launch on missiles and bombers. Negotiations begun in 2009 could further lower each nation's ceiling, to less than 1,675 deployed weapons apiece. Separately, the U.K. and France have each reduced their nuclear arsenals to about200 and 300 warheads, respectively. China has a similar number, and Israel has about 100. But no agreements exist today that would effectively stop a country producing more nuclear material for weapons. Indeed, India, Pakistan and North Korea are increasing their weapon stocks. The world needs a verifiable treaty that would help cap the buildup of nuclear weapons and make reductions irreversible everywhere.