Donald Tusk is the president of the European Council, the branch of the European Union that brings the heads of EU member states together to plan priorities. In short, his job is to be diplomatic — to figure out how to get a lot of powerful people with different worldviews to agree.

On Wednesday morning, this supreme diplomat decided to lob insults at President Donald Trump on Twitter.

“Looking at latest decisions of @realDonaldTrump someone could even think: with friends like that who needs enemies,” Tusk tweeted. “But frankly, EU should be grateful. Thanks to him we got rid of all illusions. We realise that if you need a helping hand, you will find one at the end of your arm.”

This is, to put it mildly, not how the head of the European Council typically speaks about the American president. The fact that Tusk is tweeting in this way speaks to how furious Europe as a collective is with some of Trump’s policies, most notably his decision to withdraw from the Iran nuclear deal — a decision that could end up kicking off a major trade fight between the US and Europe.

Looking at latest decisions of @realDonaldTrump someone could even think: with friends like that who needs enemies. But frankly, EU should be grateful. Thanks to him we got rid of all illusions. We realise that if you need a helping hand, you will find one at the end of your arm. — Donald Tusk (@eucopresident) May 16, 2018

But perhaps more fundamentally, Tusk’s point — that Europe can’t count on the United States anymore — shows how deeply, and possibly irreparably, Trump has damaged America’s most important alliances.

Trans-Atlantic ties depend fundamentally on predictability, on the idea that Europe could constantly count on the US to adhere to past agreements, including the NATO obligation to defend Europe in the event of an attack (from, let’s say, Russia).

Trump has called all of this into question. His policy of unpredictability, together with tearing up past US agreements, has shown Europeans that America is untrustworthy — that, even if Trump leaves office, they are always just one election away from seeing their most important alliance threatened.

That level of fear goes a long way toward explaining the vitriol you see in Tusk’s tweet.