The world’s last major U.S.–Russia nuclear arms treaty is on the brink, and the timing could not be more explosive. As New START nears its end, Washington and Moscow may soon be free from key nuclear limits just as U.S.–Iran tensions rise in the Gulf. Russia is testing next-generation nuclear systems, the U.S. is signaling new missile tests, and Iran is back in nuclear headlines. Is this the slow death of arms control and the start of a new nuclear race?