The last remaining nuclear arms control agreement between the United States and Russia—the New Strategic Arms Reduction Treaty (New START), expired on Thursday, removing the final limits on the world’s two largest nuclear arsenals.

Signed in 2010, New START capped the number of deployed strategic nuclear warheads and delivery systems held by Washington and Moscow. Its expiration marks the first time in more than 50 years that no formal restraints exist on US and Russian nuclear forces.

The treaty had been extended once in 2021 for five years. Russian President Vladimir Putin has said Moscow is willing to continue observing the treaty’s limits for another year, but US President Donald Trump has not committed to an extension.

According to White House sources, Trump wants any future arms-control agreement to include China. Beijing has rejected that proposal, arguing that its nuclear stockpile is drastically smaller than those of the US and Russia and should not be subject to the same restrictions.

The United Nations Secretary-General has described the treaty’s expiry as a “grave moment for international peace and security.”

Arms control experts warn that without New START, the risk of an unconstrained nuclear arms race increases sharply, potentially undermining global stability and making the world more dangerous.