The president of the European Council on Tuesday branded the U.S. a threat, as the backlash against the Trump administration's stance on issues such as immigration, NATO and European sovereignty intensified across the continent.
In a harshly worded open letter to 27 European Union heads of state, Donald Tusk said "worrying declarations" by the new administration were helping make the EU's future "highly unpredictable."
Tusk, who heads the council that defines EU priorities, said the external threat posed by the U.S. administration was among geopolitical conditions that include an assertive China, especially on the seas, Russian aggression toward Ukraine and its neighbors and anarchy in the Middle East and Africa, with radical Islam playing a major role.
"For the first time in our history, in an increasingly multipolar external world, so many are becoming openly anti-European, or Eurosceptic at best," Tusk wrote. "Particularly 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."
He called on European leaders to show courage and "political solidarity."
"We should remind our American friends of their own motto: United we stand, divided we fall," Tusk said.
The Tusk letter came a day after former Belgium Prime Minister Guy Verhofstadt, the European Parliament's chief negotiator as Britain prepares to leave the EU, said during a speech at a London think tank Monday that he believes Trump is trying to encourage exit referendums in Germany and France.
"My impression is we have a third front undermining the EU, and that is Donald Trump," he said.
French President Francois Hollande has called for a firm response to the new U.S. administration and a united European front, and warned Trump in a phone call Saturday against the economic and political consequences of his protectionist approach.
"Faced with an unstable and uncertain world, withdrawal into oneself is a dead-end response," Hollande said in a statement issued by the Elysee Palace.
EU leaders are scheduled to gather for a summit in Malta on Friday.
The potency of Tusk's letter is likely to have been fueled by fears over any deepening of ties between the U.S. and Russia as a result of Trump's election victory in November.
Tusk, a Polish politician, expressed concerns late last year that Russia was pursuing a strategy designed to weaken the EU and has supported keeping sanctions against Russia in place.
Across Europe, concerns have been mounting about how Trump's election will affect the EU, not least because Trump has referred to the North Atlantic Treaty Organization as "obsolete," praised Britain for voting to leave the EU and appeared to welcome the idea of other countries leaving the EU.
"I believe others will leave," Trump said in a recent interview with London's Sunday Times and German newspaper Bild. "I do think keeping it together is not gonna be as easy as a lot of people think."
In Britain, where 48% of voters favored remaining part of the European Union while 52% voted to leave in the bitterly fought referendum campaign, and more than 100,000 people recently took to the streets in a series of anti-Trump women's marches, tensions have also been running high.
British Prime Minister Theresa May is expected to attend only some elements of the Malta summit since Britain voted to leave the EU in the June referendum. Britain is preparing to extricate itself from the bloc and forge new trade alliances globally.
May has sought to establish a strong working relationship with Trump and last week became the first foreign leader to meet with him since his inauguration Jan. 20.
Her visit to Washington was touted as a diplomatic coup by the prime minister's office, and May stood with Trump in the White House and reaffirmed the strong "special relationship" that exists between the two nations, while speaking of a robust future trade deal.
She also invited Trump to a state visit hosted by Queen Elizabeth II. Hours later, the president signed an executive order temporarily banning refugees and travelers from some countries.
A petition calling for Trump's state visit to be banned because it could embarrass the queen swiftly gathered popular support and had amassed more than 1.7 million signatures by late Tuesday. The matter is to be debated in Parliament on Feb. 20.
May's public show of solidarity with Trump — and her subsequent refusal to condemn his new policies or withdraw the invitation for a state visit — has been met with skepticism and some concern.
The former head of the Foreign Office and a national security advisor, Peter Ricketts, wrote a letter in the London Times on Tuesday saying that the visit should be downgraded because it puts the queen in a "very difficult position" and was "ill-judged" advice.
"It would have been far wiser to wait and see what sort of president he would turn out to be before advising the queen to invite him," Ricketts said.
He said a possible solution could be inviting Trump for an official visit without the full pomp and ceremony of a state visit that heavily involves the monarch, and delaying the state visit until it would be viewed in a more favorable light and the queen could receive him in a celebratory, warm manner.
The queen has kept characteristically quiet about the uproar, at least in public, but her son and heir to the throne, Prince Charles, expressed his concerns during a speech at a Jewish charity's dinner Monday.
"The work of World Jewish Relief enables us to rally together, to do what we can to support people practically, emotionally and spiritually," he said. "Particularly at a time when the horrific lessons of the last war seem to be in increasing danger of being forgotten."
Charles, who campaigns about the environment, also reportedly plans to confront Trump over his views on climate change if they meet face to face, according to the Sunday Times.
Boyle is a special correspondent.