Three months after nuclear weapons leveled Hiroshima, J. Robert Oppenheimer gave voice to a dark vision. In a 16 November 1945 speech to the American Philosophical Society, he said, “If they are ever used again, it may well be by the thousands, or perhaps by the tens of thousands.” The number of US and Soviet nuclear warheads rose to extremely high levels during the Cold War, giving substance to that vision. With the end of the Cold War, however, the size of the US nuclear stockpile declined dramatically, and the number of Russian nuclear warheads is believed to have dropped in parallel. START (Strategic Arms Reduction Treaty), which came into force on 5 December 1994, and New START, which came into force on 5 February of this year, formalized limits on the strategic arsenals of the US and Russia.
Although the global nuclear stockpile is at its lowest level since 1958 (see figure 1), its destructive power remains enormous. The explosive power of each of the 4000 active US nuclear warheads is equivalent to hundreds of thousands of tons of TNT—an order of magnitude beyond the 15- to 20-kiloton yields of the warheads that destroyed Hiroshima and Nagasaki. It is therefore unfortunate that the downward impulse created by the end of the Cold War appears to be spent. In June 2013 President Barack Obama proposed to reduce the number of deployed US and Russian strategic weapons by one-third and to seek reductions in the number of nondeployed and nonstrategic weapons, but no negotiations were launched.

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