Today Russia has 4,309 deployed and stored nuclear warheads and the US has 3,700, according to the Stockholm International Peace Research Institute. The nations combined hold almost 90 percent of the global inventory. China has 600 nuclear warheads, but the figure is expected to grow to as many as 1,000 by the end of the decade.

The US and Russia stopped conducting on-site inspections of each other’s nuclear weapons facilities in March of 2020 at the start of the pandemic. Moscow suspended the New Start treaty in 2023, citing America’s leading role in the international coalition backing Ukraine.

Russia has repeatedly said it won’t rule out using unconventional weapons in response to Nato aggression, stoking fears of a nuclear war in Europe. It hasn’t made good on the threat, but since starting the war Russia has twice fired an Oreshnik missile into Ukraine. The missile can carry conventional or nuclear warheads.

There is currently no arms control treaty in place restricting the use of shorter-range “tactical” nuclear warheads in Europe.

Last month, Trump signalled he was comfortable with letting the treaty expire. “If it expires, it expires,” he said in an interview with the New York Times. “We’ll just do a better agreement.”

Russia said on Wednesday ahead of the treaty’s expiration that it was “no longer bound” by New Start restrictions on deployed nuclear warheads.

“We assume that the parties to the New Start treaty are no longer bound by any obligations or symmetrical declarations within the context of the treaty,” Russia’s foreign ministry said in a statement.

Even if both sides reach a stopgap agreement, experts said the prospects of a long-term arms control treaty remained murky.

“There could be a political handshake agreement to extend New START’s central limitations [but] there is no indication that that would include verification measures,” said Stephen Herzog, a professor at the James Martin Center for Nonproliferation Studies.

Years of arms control policy now hang in the balance, Herzog added. The lapse of New Start, he said, disrupted “a legacy of more than five decades of bilateral nuclear arms control.”