Read: Hong Kong shows the flaws in China’s zero-sum worldview

During Hong Kong’s 2014 umbrella protests, which were not as far-reaching as those taking place now, President Barack Obama struck a cautious note, expressing America’s inherent sympathy for freedom of speech and association and saying his government’s primary message was the avoidance of violence. Republicans, including Senator Marco Rubio, criticized Obama for not being more supportive of the protesters. The White House worried that any support would lend credibility to Beijing’s claim that the protests were orchestrated by the United States—and was careful not to overpromise.

Presidents are constrained in what they can say. We should cut Donald Trump some slack. But even taking those constraints into account, Trump’s response could hardly have been worse. Not only was Trump silent on America’s core values. He also increased the risk of a major miscalculation by China with seismic geopolitical consequences. It may prove to be the greatest mistake of his presidency.

Trump’s folly began with a phone call to China’s president, Xi Jinping, on June 18. According to the Financial Times and Politico, Trump told the Chinese leader that he would not condemn a crackdown in Hong Kong. The commitment was made on the fly, without prior consultation with his national-security team. On August 1, Trump made good on that secret promise when he told the press:

“Something is probably happening with Hong Kong, because when you look at, you know, what’s going on, they’ve had riots for a long period of time. And I don’t know what China’s attitude is. Somebody said that at some point they’re going to want to stop that. But that’s between Hong Kong and that’s between China, because Hong Kong is a part of China. They’ll have to deal with that themselves. They don’t need advice.”

Photos: Massive protests in Hong Kong

On August 13, Trump called it “a very tough situation,” but added that he hoped “it works out for everybody, including China.” Later he tweeted, “Many are blaming me, and the United States, for the problems going on in Hong Kong. I can’t imagine why?” His secretary of commerce, Wilbur Ross, told CNBC, “What would we do, invade Hong Kong? … It’s a question of what role is there for the U.S. in that manner? This is an internal matter.” Politico reports have quoted administration officials as saying that Trump is singularly focused on a trade deal and does not want human rights to get in the way. After a torrent of criticism, and a deluge of statements from congressional leaders, he tweeted last night, vaguely asking Xi to deal with Hong Kong “humanely” and hinting at a meeting. He could have said no violence, but he chose not to. This morning, he again praised Xi and suggested he meet with protesters. It was marginally better than the unconditional green light he had offered previously, but it is still far short of what is required.