Trump flails, but he’s right that China can’t join all the right international clubs and go on playing by its own rules. It can’t make some trade “deal” and then not be held fully accountable, relying on the infinite global capacity to turn a blind eye to its predations.

Ordering American companies out of China one day — a trademark Trump grotesquerie — and praising Xi the next is no way to negotiate, but China has had a pass for too long. The president’s statement linking a trade deal and the Hong Kong demonstrations — “It would be very hard to deal if they do violence. I mean, if it’s another Tiananmen Square, it’s — I think it’s a very hard thing to do if there’s violence” — was perhaps his finest hour. I just hope he meant it, despite the contempt he has otherwise shown for human rights.

A lot is at stake in Hong Kong, the world’s third-largest capital market. It’s a pivotal moment of the American-Chinese ideological war. A second Tiananmen could turn Xi’s overreach into cataclysm. He’s more vulnerable than he looks — as Trump intuits. Right now, of the Democratic presidential candidates, only Elizabeth Warren appears to get the China threat, one reason she’s surging. China will be big in 2020.