It’s no secret that U.S. President Donald Trump likes complaining about his NATO allies, who he frequently accuses of taking the United States for a ride. He has even gone so far as to say that he’d “encourage” Russian President Vladimir Putin to attack NATO members who don’t pay their way.

As alarming as the numerous anti-NATO outbursts from the White House have been, they are seldom followed by drastic action. But one of the president’s lesser-discussed criticisms of NATO is particularly persistent—and dangerous.

It’s no secret that U.S. President Donald Trump likes complaining about his NATO allies, who he frequently accuses of taking the United States for a ride. He has even gone so far as to say that he’d “encourage” Russian President Vladimir Putin to attack NATO members who don’t pay their way.

As alarming as the numerous anti-NATO outbursts from the White House have been, they are seldom followed by drastic action. But one of the president’s lesser-discussed criticisms of NATO is particularly persistent—and dangerous.

Trump does not approve of NATO’s open-door enlargement policy. He has argued that it helped provoke Russia’s invasion of Ukraine and has repeatedly insisted that Kyiv will not join the alliance on his watch. Even many of Ukraine’s strongest supporters now concede that its path to membership is effectively blocked.