President Donald Trump’s statement indicating that the United States will restart nuclear weapons testing has overturned decades of U.S. policy and alarmed nonproliferation experts who warn it could trigger a new global arms race.

The announcement came Thursday ahead of Trump’s meeting with Chinese President Xi Jinping. In a Truth Social post, Trump said he had directed the “Department of War” to resume nuclear testing “on an equal basis” with other nations — despite nuclear oversight falling under the Energy Department, not the Defense Department.

The U.S. last conducted a nuclear test in 1992 at the Nevada National Security Site. Since then, it has relied on computer modeling and simulations to maintain its arsenal. The country signed the Comprehensive Nuclear Test-Ban Treaty in 1996 but never ratified it.

Experts say Trump’s move represents the most significant reversal in U.S. nuclear policy since the end of the Cold War. They warn it could prompt Russia, China, and other nuclear powers to conduct their own tests, destabilizing global security.

Russia recently tested its Burevestnik cruise missile — known to NATO as Skyfall — and a nuclear-powered underwater drone, while China continues to expand its missile silos. North Korea has also unveiled a new intercontinental ballistic missile capable of reaching the U.S. mainland.

Trump claimed the U.S. holds more nuclear weapons than any other country, though data from the Arms Control Association indicates Russia has 5,580 warheads, compared to America’s 5,225. Together, the two nations possess nearly 90% of the world’s nuclear stockpile.

Restarting testing in Nevada would require extensive site preparation, as the facilities have been inactive for more than three decades. A 2018 Los Alamos National Laboratory briefing estimated that even a single test could take up to four years to plan and execute.

Analysts say the decision, if carried out, would be political rather than scientific and could reignite nuclear tensions not seen since the Cold War.