President-elect Donald Trump unexpectedly tweeted in support of boosting the United States’ nuclear capacity on Thursday, marking a potential reversal in U.S. policy.

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Thursday’s tweet came “hours after Russian President Vladimir Putin pledged to enhance his country’s nuclear forces,” CNN reports. It drew criticism from experts like Jeffrey Lewis, the director of the East Asia Nonproliferation Program at the Middlebury Institute of International Studies, who told Business Insider that nuclear proliferation would be a “total catastrophe” for the U.S. and its allies abroad.

Citing Lewis, Business Insider explained:

Others responded on Trump’s preferred medium expressing similar fears.

Trump’s previous statements on nuclear arms have been inconsistent at best.

As such, during the presidential election, the No Red Button campaign warned of the dangers of Trump having access to nukes.

“Donald Trump is weeks away from having the unchecked ability to light the world on fire,” Meredith Horowski, the director of the campaign, told Common Dreams in October. “His flippant statements condoning nuclear violence and his penchant for revenge suggest that as president, he would do just that. He could launch thousands of nuclear weapons at any time—each one vastly more powerful than the bomb that obliterated Hiroshima—and no one could stop him.”