The last nuclear arms control pact between the United States and Russia is set to end not with a bang, but with a whimper. U.S. government hawks believe that fielding as many nuclear weapons as Russia and China combined would hold both in check. It does not cover Russia’s sizable stockpile of so-called tactical nuclear weapons, which are shorter-range, nor does it cover Russia’s new and more exotic nuclear weapons, like the ocean-spanning Poseidon submarine drone. It’s strange that Mr. Trump has not taken up Mr. Putin on his offer for adhering to the treaty’s limits. The American president has long lamented the trillions of dollars collectively spent on nuclear weapons by the world’s superpowers.