The European Union has been struggling to contend with fractious internal forces. Among them: the vote by Britain to leave the bloc, the organization’s failure to establish a unified response to the arrival of hundreds of thousands of asylum seekers and the debt crisis that has driven many Greeks into poverty. And then there are external pressures like Russia’s annexation of Crimea.
Before the election and since taking office, Mr. Trump has lauded the vote by Britain, known as Brexit, and said the country would thrive outside the European Union. He met with Nigel Farage, a populist leader of the campaign to leave the bloc, before seeing Prime Minister Theresa May. And at one point he went so far as to suggest that Mrs. May appoint Mr. Farage as Britain’s ambassador to the United States.
Mr. Trump has also praised President Vladimir V. Putin of Russia and indicated he would pursue friendlier relations with Moscow, even as Russia encourages chaos on the European Union’s eastern border.
Mr. Tusk’s letter does not reflect a new policy for the European Union, and member states of the 28-nation bloc are not required to act on Mr. Tusk’s advice when they meet on Friday. But many European leaders have made their differences with Mr. Trump known.
After the United States said it was temporarily blocking refugees from entering the country, Chancellor Angela Merkel of Germany felt compelled to point out to Mr. Trump the obligations of nations under the Geneva Conventions to protect refugees of war on humanitarian grounds. And President François Hollande of France said he had reminded Mr. Trump that “the ongoing fight to defend our democracy will be effective only if we sign up to respect to the founding principles and, in particular, the welcoming of refugees.”
Mrs. May, of Britain, sought in a meeting with Mr. Trump last week to confirm his commitment to NATO; he was dismissive of the alliance, the bedrock of European security, during his campaign.
Now, the sentiments expressed in Mr. Tusk’s letter are pushing European leaders’ exasperation with the American president further into the public view.
Mr. Tusk has sounded the alarm about the existential crises facing the bloc before, but never with the urgency he displayed in the letter. And he has never before included a longstanding ally like the United States in the list of challenges.
“An increasingly, let us call it, assertive China, especially on the seas,” he wrote, “Russia’s aggressive policy toward Ukraine and its neighbors, wars, terror and anarchy in the Middle East and in Africa, with radical Islam playing a major role, as well as worrying declarations by the new American administration all make our future highly unpredictable.”
Much of the frustration Mr. Tusk displayed in his letter stemmed from what Guntram B. Wolff, director of Bruegel, a research organization in Brussels, said was Mr. Trump’s “de facto supporting” of populist forces that could further upend the European order.
Far-right populist challengers in France, Germany and the Netherlands have adopted some of his anti-establishment rhetoric in their own campaigns.
Still, Mr. Wolff said it was unwise to enter into a war of words with the Trump administration. “We need to uphold our values here, but does it mean that we need now a declaration where we put the United States on the same level as ISIS?” he said. “No, I don’t think so. I don’t think it that would be helpful in any way.”
The trans-Atlantic volley of opprobrium on Friday included an accusation by Peter Navarro, the director of Mr. Trump’s new National Trade Council, that Germany was manipulating its currency to gain a trade advantage. Mr. Navarro told The Financial Times that Germany was using a “grossly undervalued” euro to “exploit” the United States and its partners in Europe.
That did not sit well with Ms. Merkel, who defended the European Central Bank’s independent role at a news conference on Friday: “Because of that we will not influence the behavior of the E.C.B. And as a result, I cannot and do not want to change the situation as it is.”
The value of the euro is near a 13-year low compared with the dollar, allowing German carmakers and other manufacturers to sell their goods more cheaply in the United States. But German firms also employ around 670,000 people in the United States, including many in a BMW factory in Spartanburg, S.C., the carmaker’s largest in the world, and a Mercedes factory in Tuscaloosa, Ala. These are the sort of manufacturing jobs that Mr. Trump says he wants to keep in the United States.
Jan Techau, director of the Richard C. Holbrooke Forum in Berlin, a research center dedicated to diplomacy, said Mr. Tusk’s letter was less a warning to the American president than it was a message to Europeans not to be lured away from union, or to be tempted away from the bloc by favorable bilateral ties offered by the Trump administration. “He is encouraging everyone to fall into that trap,” Mr. Techau said of the American president.
Mr. Tusk, by contrast, is making the case for Europeans to stick together for their own survival. “He wants to remind them that there is something bigger at stake than just what they are going to be talking about in Malta,” Mr. Techau said.