U.S.-Russia nuclear weapons treaty has expired. What happens now?

The treaty between the U.S. and Russia limiting their deployment of strategic nuclear weapons has expired, leaving plenty of questions about what U.S. President Donald Trump and Russian President Vladimir Putin intend to do next.

The expiration of New START — the latest in a series of nuclear weaponry limitation deals between Washington and Moscow dating back to 1972 — means that for the first time in more than 50 years, no formal agreement is, in effect, capping the size of the world's two largest nuclear arsenals.

While experts disagree on whether the treaty's end will kick off a new nuclear arms race, there's broad consensus that the deal needs significant updating to reflect current nuclear realities, including the growth of China's weapons program.

Yet the hope that Trump, Putin and China's president, Xi Jinping, will agree on such an update quickly appears idealistic at best.

Thomas Countryman, a former U.S. diplomat who served as acting under-secretary of state for arms control and international security, says the risk that a nuclear weapon could be used in an attack is now the highest it's been since the Cuban Missile Crisis of 1962.

"That's why we watch with alarm as this treaty expires," Countryman told CBC Radio's The Current.

Under the now-expired treaty, the U.S. and Russia agreed to deploy no more than 1,550 nuclear warheads each and to cap their deployments of intercontinental and submarine-launched ballistic missiles and launchers.

Last August, in the run-up to his summit with Trump in Alaska, Putin dangled the idea of renewing the treaty, a proposal that never came to fruition.

The treaty was by then partially undercut anyway: In 2023, Russia suspended its adherence to the agreement's terms in retaliation for the Biden administration's support of Ukraine.

Trump recently brushed off the significance of the treaty's looming expiration, insisting he can get a better agreement in place.

On Thursday, he described New START as "a badly negotiated deal" in a social media post.

"The president wants to have our nuclear experts work on a new, improved and modernized treaty that can last long into the future, and that's what the United States will continue to discuss with the Russians," White House Press Secretary Karoline Leavitt told reporters on Thursday.

U.S. Secretary of State Marco Rubio is also downplaying the treaty's expiration and emphasizing the need for bringing in China.

"The president’s been clear in the past that in order to have true arms control in the 21st century, it’s impossible to do something that doesn’t include China because of their vast and rapidly growing stockpile," Rubio told reporters in Washington on Wednesday.