The world's two largest nuclear powers, Russia and the United States, no longer have any limits on their arsenals.

At midnight on Thursday, a 15-year-old treaty called New START expired, and with it, caps on the number of weapons the two sides could deploy on missiles, bombers and submarines.

"There are no more guardrails on the sizes of the United States and Russian strategic nuclear arsenals," said Christine Wormuth, president of the Nuclear Threat Initiative, an arms control advocacy group based in Washington, D.C. "That has not been the case for decades."

President Trump posted on social media on Thursday, "Rather than extend 'NEW START' (A badly negotiated deal by the United States that, aside from everything else, is being grossly violated), we should have our Nuclear Experts work on a new, improved, and modernized Treaty that can last long into the future."

The White House had told NPR earlier that Trump would like to "involve China in arms control talks."

Addressing the impending demise of the treaty earlier this week, Kremlin spokesman Dmitry Peskov warned that it would be a "more dangerous" world without limits on U.S. and Russian nuclear arsenals.

New START was negotiated with Russia under President Barack Obama. At the time, the treaty was just the latest in a 50-year effort to bring down the number of nuclear weapons each side pointed at the other. ("New START" stands for New Strategic Arms Reduction Treaty. It replaced a previous START I treaty signed by President George H.W. Bush and Mikhail Gorbachev in 1991.)

At the height of the Cold War, those numbers were believed to be around 12,000, said Rose Gottemoeller, a professor at Stanford University who led the New START negotiations. "The New START treaty brought [the U.S. and Russia] down to 1,550 deployed warheads."

But New START did more than just put limits on warheads. The treaty established a whole system by which the U.S. and Russia notified each other every time they moved a nuclear weapon.

"Over the 15-year lifetime of the treaty, more than 25,000 of those notifications were exchanged," said Matt Korda, associate director of the Nuclear Information Project at the Federation of American Scientists.

The two sides even sent inspectors to nuclear sites.

"We were going to each other's missile bases and bomber bases, submarine bases and checking up that we were each abiding by our commitments under the treaty," said Gottemoeller.

The limits, notifications and inspections are all credited with creating a lot of stability between the world's two largest nuclear powers, Korda said.

"There are a lot of good things that both Russians and Americans can say about New START," says Dmitry Stefanovich, a research fellow at the Primakov Institute in Moscow.

"It was a good treaty and it did a lot of good for both countries and for global security."

That doesn't mean Russia wants to rush into another arms race with America, says Pavel Podvig, an expert on Russian strategic nuclear forces at the United Nations Institute for Disarmament Research in Geneva.

Due to the war in Ukraine, Russia is strapped for cash and not particularly interested in building up its arsenal, Podvig says. For the government, nuclear weapons are more a matter of pride.

"Russia wants to be an equal partner to the United States," he said. Its nuclear arsenal is one way it can claim parity.

Last fall, Russian President Vladimir Putin offered Washington a one-year informal extension of the limits in New START to give negotiators more time to work out a formal agreement.

President Trump still hasn't taken Putin up on his offer — a nonresponse Russia's Foreign Ministry lamented.

"In fact, it means that our ideas have been deliberately left unanswered. This approach seems erroneous and regrettable," the Foreign Ministry said in a statement.

"My biggest concern is that we could end up engaging in an unconstrained nuclear arms race with both Russia and China at the same time," Wormuth said. China has for years kept a much smaller arsenal than the U.S. and Russia but it recently began rapidly expanding its nuclear forces with a goal of becoming a strategic equal to those two powers.

Wormuth, a former secretary of the Army, says just as for Russia, the cost of an arms race would weigh on the United States.

Building up the nuclear arsenal is "going to be very, very expensive at a time where our national debt is tremendous. We have a lot of pressing priorities domestically that need investment, and frankly the United States' conventional military needs investment," she said.

In fact, the U.S. is undergoing a major upgrade to the nuclear weapons it already has. Over the next decade, just those upgrades are projected to cost about a trillion dollars.

But, with New START's demise, the prospect of a future arms race has moved closer.

"It's not like the arms race will begin on February 6th," says the Primakov Institute's Stefanovich.

"But if we don't have any limitations and we don't have negotiations, both countries will plan for the worst case scenario," continued Stefanovich.

In his view, that meant both sides hedging against a strategic imbalance by building up their arsenals.

Meanwhile, the U.S. is worried about its nuclear rivals, especially China. Speaking on Wednesday, Secretary of State Marco Rubio said the government would not consider arms control talks unless China participated.

"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 without China," he said.

Given all the global uncertainty, Wormuth worries that the U.S. could eventually begin to reconsider the size of its arsenal.

If that were to happen, there would likely be new requirements to deploy more nuclear weapons in order to target new Chinese and Russian sites, she warned.

"Pretty soon you find yourself in a very strange world of nuclear weapons logic," she said, adding that logic often makes more seem better. (NPR)

•PHOTO: In the above photo made from footage taken from the Russian Defense Ministry official website on May 24, 2018, a Russian nuclear submarine test-fires Bulava missiles from the White Sea on May 22, 2018. (Credit: Russian Defense Ministry Press Service/AP)

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