There are four components to the external threat facing the European Union, according to European Council President Donald Tusk: China, Russia, "terror and anarchy in the Middle East and in Africa" and . . . the United States.

Ahead of a summit in Malta on the future of the EU, Tusk sent a letter to the 27 EU heads of state calling on them to stand together in increasingly challenging times - and calling the US a threat to Europe.

"The change in Washington puts the European Union in a difficult situation; with the new Administration seeming to put into question the last 70 years of American foreign policy," Tusk wrote, citing "worrying declarations by the new American Administration".

The letter caused some in US foreign policy circles to sit up, but its intended audience was Europeans.

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"The new president has a hostile approach toward Europe," said Stefan Lehne, visiting scholar at Carnegie Europe. He said he was troubled by Trump's attitude because the European Union has, for essentially its entire existence, relied on and turned to the United States as an ally.

From the 1950s onward, every president - even during tense or tumultuous times - viewed European integration positively. Trump, on the other hand, congratulated the United Kingdom on Brexit, and seems not to mind if the EU falls apart.

Lehne noted there are some experienced and pragmatic players on Trump's team. But he questioned whether they would have the President's ear and worried that he might try to play European heads of state off of one another.

And what will the latter mean for the European Union? "If it doesn't pull together, it will pull apart," Lehne said. Pulling together, he said, means more investing in foreign policy and defence, promoting active trade policy and "putting the house in order in terms of migration."

But all of that is, of course, easier said than done.

- Foreign Policy