“If our American partners suspend their participation in the I.N.F. treaty, so will we,” he said on Feb. 2. “And if they start working on new weapons, so will we.”

It’s unlikely the treaty will be revived, given the deterioration of Russian-American relations, despite Mr. Trump’s promises of warmer ties and a coziness that has drawn the scrutiny of the special counsel Robert Mueller.

The pact’s imminent collapse has intensified concerns that the United States and Russia will let the 2010 New Start agreement, with its caps on deployed nuclear warheads, intercontinental ballistic missiles and heavy bombers, as well as requirements for verification and data exchanges, expire in 2021. (Start stands for Strategic Arms Reduction Treaty.) Absent an extension or a new treaty, the nuclear arsenals will become unregulated — meaning there will be no legally binding, verifiable limits on the American or Russian nuclear arsenals — for the first time since 1972.

Adding to the uncertainty, Mr. Putin has warned that Russia is developing new “invincible” hypersonic missiles that will travel at more than five times the speed of sound.

The United States, meanwhile, is developing a new conventionally armed intermediate-range missile that could one day be deployed in Europe. By the end of the year , Washington may be ready to field a low-yield nuclear weapon that could make it easier to use nuclear weapons without courting Armageddon.

Last month , Mr. Trump threatened to unsettle the nuclear landscape even further by promising to reinvent American missile defenses, a quixotic vision reminiscent of President Ronald Reagan’s unfulfilled “Star Wars” program.

Mr. Reagan appreciated the dangers of nuclear war and the value of negotiated agreements that could reduce weapons. The New Start numbers can go even lower. It has been over seven years since the two powers held serious nuclear arms reductions talks. The United States, its allies, even the Russians will be less safe if that process atrophies.