The New START agreement between the United States and the Russian Federation, in force since 2011, puts caps on the number of American and Russian “strategic” weapons, the long-range missiles and bombers that can cover the thousands of miles between North America and Eurasia. It is the last in a line of treaties that helped stabilize the relationship between the superpowers during the the tense years of the Cold War, and then provided the framework for serious reductions in nuclear weapons after the fall of the Soviet Union. On Thursday, the two largest nuclear powers will be free to begin a new arms race, a needless competition that both nations have managed to avert for decades.