U.S. President Donald Trump, so amenable to Russia on so much, has been talking tough when it comes to nuclear weapons.

On Thursday, Reuters reported that in his phone call last month with Russian President Vladimir Putin, Trump criticized the 2010 New START treaty, an Obama-era agreement designed to limit both sides’ nuclear arsenals. Putin reportedly asked about extending the treaty beyond 2021. Trump, asking after aides what New START was, said he thought the treaty was a bad deal.

This apparent willingness to jettison a successful arms limitation treaty was not received well by groups worried about nuclear war. David Wright, co-director the Global Security Program at the Union of Concerned Scientists, said in a statement, “The provisions apply equally to both nations. It’s a win-win situation in which both nations—as well as the rest of the world—are more secure. It would be a grave mistake for the United States to pull out of the treaty or allow it to expire.”

Bruce Blair, co-founder of Global Zero, a group dedicated to limiting proliferation, released a statement saying, “The Treaty is also a powerful restraint on Putin’s nuclear ambitions. Without it, there would be nothing to prevent Russia from expanding its nuclear arsenal after 2021. That could trigger a dangerous nuclear arms race, one Trump recently promised to win no matter the cost.”

What’s not clear is exactly what Trump’s stance on nuclear weapons is. During the campaign, he mulled using nukes on the battlefield and encouraged allies like Japan and South Korea to build the bomb. After taking office, he said the United States needs to increase its nuclear capabilities, telling the hosts of MSNBC program Morning Joe, “Let it be an arms race.”

At other times, Trump suggested easing sanctions on Russia in exchange for nuclear arms reductions. “For one thing, I think nuclear weapons should be way down and reduced very substantially, that’s part of it,” he told the Times of London, adding, “But Russia’s hurting very badly right now because of sanctions, but I think something can happen that a lot of people are gonna benefit.”

Perhaps, then, Trump’s rejection of the extension of New START and threat of an arms race are just negotiating tactics. If that’s the case — if Trump is attempting to rewrite the art of the nuclear deal — there are still several big challenges.

For one thing, Russia has already rejected the idea. On the day after Trump’s inauguration, Kremlin spokesperson Dmitri Peskov said symmetrical disarmament between Russia and the United States was “unacceptable.” And the Kremlin could easily call Trump’s bluff: Russia has on more than one occasion conducted mock nuclear attacks in its war games; boycotted last year’s Nuclear Security Summit; and deployed nuclear capable missiles to Kaliningrad.

But maybe Moscow is just playing hard to get. Even so, there’s another issue: Europe and NATO. A U.S.-Russia arms race does not benefit Europe, and European leaders would therefore support a deal reducing nuclear weapons — but they have to be at the table.

“If there is a Trump-Putin negotiation on anything – including nuclear – that is over the heads of European leaders, that would be very damaging for NATO cohesion and transatlantic security relations,” said Sir Adam Thomson, Director of the European Leadership Network.

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