But now, killing treaties has become another form of escapism. After the Sept. 11 attacks, the George W. Bush administration announced America’s withdrawal from the Antiballistic Missile Treaty signed between the two superpowers in 1972. Mr. Putin’s response was to walk away from a 1993 Strategic Arms Reduction Treaty that prohibited land-based missiles from carrying multiple warheads, or MIRVs. Eliminating these missiles was a long-sought goal of arms control. Now Russia is deploying MIRVed missiles in the worst possible way — with easily targeted, slow-to-use liquid-fueled missiles based in silos — conditions that could provoke a launching at the first suspicion of an attack, even if unverified, for fear that they would be destroyed .

In August, Moscow and Washington seem set to leave the Intermediate-Range Nuclear Forces Treaty signed by Ronald Reagan and Mikhail Gorbachev in 1987 — the treaty that broke the back of the Cold War nuclear arms race. Mr. Trump has announced America’s withdrawal without seeking to negotiate remedies to Russian violations. In his view, nothing less than destroying the offending Russian missiles would do. But now, Mr. Putin is free to deploy more of them.

Next up is “New Start,” the sole surviving Russian-American treaty that limits longer-range missiles. It could be extended for another five years, but Mr. Trump and his national security adviser, John Bolton, have neither the diplomatic skills nor apparently an interest in extending its terms or negotiating better ones. On top of this, Mr. Trump and Mr. Bolton have championed new missile defense plans to protect every American city from any missiles from any origin.

The problem with such plans is that ground-based missile defense interceptors remain woefully unable to do this — they don’t have enough time to catch up to their targets. The only theoretical hope to protect American cities lies in space-based interceptors. But that pursuit, like President Reagan’s Strategic Defense Initiative, is most likely to again fail for technical, financial and political reasons. If the United States deploys interceptors in space, Russia and China will probably pursue countermeasures in the form of space mines that could trail American space-based interceptors, at a small fraction of the cost. In other words, the hair trigger of terrestrial nuclear warfare would simply be duplicated in space.

How did we manage to survive the seven harrowing decades since Hiroshima and Nagasaki without the battlefield use of nuclear weapons? Deterrence alone didn’t produce this result; the combination of deterrence and diplomacy did. This safety net was the greatest unacknowledged achievement during the Cold War. It now has very little load-bearing capacity left.