IT IS not very often that the Kremlin issues a transcript of remarks by President Vladimir Putin with a sentence marked in highlighter, but that’s what happened Dec. 5 when Mr. Putin met with leaders of Russia’s defense industry. The highlighted sentences said Russia is willing to renew the New START nuclear weapons treaty immediately, before the year is out, and without any preconditions. This is an offer that President Trump ought not refuse.

The New START treaty took effect Feb. 5, 2011, and expires in February 2021 unless both sides act to extend it for no more than five years. Both sides met the 2018 deadline to reduce their strategic nuclear forces to no more than 1,550 warheads on 700 deployed missiles and bombers. The treaty included a detailed and valuable verification regime. It has clearly been in the interests of the United States, providing a cap on the most dangerous nuclear weapons stockpiles and a sense of predictability about the future. It does not constrain the U.S. modernization program now underway; the new systems are replacing older ones, remaining within the treaty limits. A five-year extension seems to be a no-brainer.

That’s why Mr. Trump’s indifferent approach to the extension is so puzzling. Russian officials have been raising the prospect of an extension for more than a year. The Trump administration does not appear to have engaged Moscow in any serious negotiations toward an extension.

Instead, Mr. Trump and others have broached the idea of a new treaty with broader aims, to include the nonstrategic nuclear weapons that are currently not under any treaty, and to include China, a nuclear weapons power that is not part of the New START treaty. Both goals are laudable in principle. However, a treaty covering so-called nonstrategic or tactical nuclear weapons will be complex and take time. Is Mr. Trump actively and seriously preparing for negotiations? There is no sign of it.

The China equation is even more difficult. China’s nuclear arsenal is much smaller than either the United States’ or Russia’s. According to the Congressional Research Service, China has about 130 warheads on missiles that would count under New START, out of a total of about 280 warheads. In the past it has not kept nuclear warheads on launch-ready alert, as do Washington and Moscow. China has been adamant that it does not intend to join arms-control negotiations. There is no evidence Mr. Trump has made any serious effort to talk to China about nuclear weapons reductions, and given the current tensions with Beijing over trade and human rights, it would seem problematic.

So what is Mr. Trump’s strategy? Is he using the China angle as a poison pill to avoid extension of New START and cause the treaty to expire? If so, it would mark yet another plank crashing from the already-rickety structure of arms-control treaties intended to restrain the nuclear danger. New START is the last of the major nuclear agreements still in force, and it should be given five more years.

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