Probably everyone who followed Donald Trump’s visit to Britain has a favorite scene of diplomatic debacle. But the moment that probably did the most to poison relations with our oldest ally — and undermine whatever chance there was for the “phenomenal” trade deal Trump claimed to be offering — was Trump’s apparent suggestion that such a deal would involve opening up Britain’s National Health Service to U.S. private companies.

It says something about the qualities of our current president that the best argument anyone has made in his defense is that he didn’t know what he was talking about. He does, however, know what the N.H.S. is — he just doesn’t understand its role in British life.

After all, last year he tweeted that Britons were marching in the streets to protest a health system that was “going broke and not working.” Actually, the demonstrations were in favor of the N.H.S., calling for more government funding.

But never mind what was going on in Trump’s mind. Let’s focus instead on the fact that no American politician, Trump least of all, has any business giving other countries advice on health care. For we have the worst-performing health care system in the advanced world — and Trump is doing all he can to degrade it further.