The deadline has been looming over Washington and Moscow for years.

On Thursday, the last nuclear arms control treaty between the United States and Russia expired. For the first time since 1972, it leaves both superpowers with no limits on the size or structure of their arsenals, at the very moment both are planning new generations of nuclear weapons and newly evasive means of delivering the deadly warheads.

Despite a new era of superpower confrontation, talks over a new treaty — or even an informal extension of the current one — never got off the ground, frozen by the war in Ukraine. When President Trump was asked in January why he had not taken up President Vladimir V. Putin’s offer for a one-year informal extension, he shrugged.

“If it expires, it expires,” he told The New York Times in an interview. “We’ll do a better agreement” after the expiration, he insisted, adding that China, which has the world’s fastest-growing nuclear arsenal, and “other parties” should be part of any future accord. The Chinese have made clear they are not interested.

On Thursday afternoon, after the New START treaty’s expiration, Mr. Trump reiterated his call for a new accord, denouncing the previous one as “a badly negotiated deal” and declaring on social media that “we should have our nuclear experts work on a new, improved, and modernized treaty that can last long into the future.” But he said nothing about agreeing with Mr. Putin to freeze American and Russian arsenals at current levels, leaving open the possibility of a renewed arms race.

In fact, the United States has been preparing for that possibility, and the Navy is already preparing to deploy more nuclear warheads on its biggest submarines. Meanwhile, Russia and China are now testing new types and configurations of nuclear weapons that few envisioned when the Senate, by a narrow margin, ratified New START in 2010.

Arms control was not supposed to end this way.

When President Richard M. Nixon signed the first arms limitation treaty with the Soviet Union, the banner headlines signaled a new era in which even the most hostile of Cold War rivals saw the danger of letting the arms race spin out of control.