(CNN) The European Union declared the Trump administration a "threat" on Tuesday, laying bare what many Europeans think privately and setting the stage for increased tension between the US and EU.

European Union President Donald Tusk's diplomatic bombshell listed the Trump administration as a threat alongside China, Russia, terrorism and radical Islam, adding that "worrying declarations by the new American administration all make our future highly unpredictable."

"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 said in a letter to EU members.

The astonishing break from diplomatic practice stems from reasons that range from the personal to the broadly geopolitical.

Tusk's stark description about a close ally of seven decades reflects deep unease about President Donald Trump's take on European institutions. He's called NATO "obsolete," dismissed the 28-member EU as a "vehicle for Germany" and publicly said he's had "a very bad experience" with the EU as a businessman.

There is concern that Trump's comments will not only undermine the EU, but benefit Russia, which would prefer a weakened NATO and a strained Europe-US alliance.

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And then there is deep wariness about Trump's chief strategist Stephen Bannon -- not just because of his anti-EU views and influence on the President but because his website Breitbart News is looking to expand into Europe. Diplomats said there's concern the site's cocktail of fake news and conspiracies could impact upcoming European elections.

"Tusk's letter speaks to one challenge Europeans see -- Trump's skepticism," said Fran Burwell, a distinguished fellow at the Atlantic Council. "But there's another challenge in Bannon," who is close to populist European politicians such as France's Marion le Pen and her aunt Marine, leader of the National Front.

While some experts champion a move away from the EU and multilateral organizations, many diplomats and analysts said the new US administration seems to be trying to rewrite the terms of the US-EU alliance in ways that are potentially destabilizing.

Tusk's "dramatic language is something you wouldn't expect. It's extremely worrying, but I can see why. Trump's policies to the EU are completely unprecedented," said Stefan Lehne, a former EU diplomat from Austria now with Carnegie Europe. "Every Brit and European was socialized to expect the US to lead on every international crisis. Now you have a US president who wouldn't mind at all if the EU fell apart."

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Lehne notes that Tusk's statement comes as the EU faces Russian assertiveness, a refugee crisis, rising populist movements in Europe, and critical elections in France, the Netherlands and possibly Italy.

"There's a lot at stake and all these negative dynamics amount to a crisis. Tusk seems to feel if all this comes together, if the EU doesn't come together, it will come apart. It is really a difficult moment."

Burwell describes it as "really earth shattering for many. It's a fundamental challenge."

Nile Gardiner of the Heritage Foundation agreed it's a "sea change," but sees it as a positive.

"The old arguments in favor of European integration no longer apply," he said. "The winds of change are sweeping through Europe with a drive toward sovereignty, self-determination, decentralization of power. Donald Tusk is in a state of denial as to the trajectory in which Europe is moving. President Trump has a better understanding."

Trump shows little love for the EU, saying at a Friday press conference with British Prime Minister Theresa May that he had a "very bad experience" in which "getting the approvals from Europe was very, very tough." Trump seemed to be referring to an EU ruling against a wall he wanted to build at an Irish golf course he owns because it would endanger protected snails.

The State Department referred requests for comment about Tusk's letter to the White House, which did not respond. The EU mission to the US said Tusk's letter speaks for itself.

In that letter inviting member states to a meeting on Friday, Tusk said, "We cannot surrender to those who want to weaken or invalidate the Transatlantic bond, without which global order and peace cannot survive. We should remind our American friends of their own motto: United we stand, divided we fall."

Derek Chollet, a senior adviser for security and defense at the German Marshall Fund, said a divided Europe and a weaker US-Europe relationship could make it harder for the US to find partners to work with -- particularly on global security issues -- there could be economic fallout that hurts US businesses, and it could leave Russia "empowered and getting everything it wants -- a US divided from Europe and an EU that is weakened and perhaps breaking apart -- without having to do anything. "

"To the extent that Trump seeks to undermine or weaken the EU, that benefits Russia," said Chollet. "This is a softball pitch over the plate to (Russian President) Vladimir Putin."

Gardiner, of the Heritage Foundation, said Russia would prefer to deal with a weak EU than individual countries. "Sovereign nations can do more than the lowest common denominator," Gardiner said.

Burwell added that one of May's messages for Trump was a request not to weaken the EU. "They are cooperating very strongly with Europe, the EU, in terms of sharing intelligence -- when she was Home Secretary she was central to that," said Burwell, "so the message was 'we're leaving but we still want them strong.' "

While the administration hasn't yet articulated a policy that would actively undermine the EU, Chollet said, "the fact that Trump has embraced people like Nigel Farage," the leader of the Brexit movement "who seeks to undermine the EU, and that advisers like Steve Bannon are on the rise, it's leaving Europeans asking questions whether the US is a reliable ally."

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Several diplomats said anxiety is running high in Europe, with leaders quietly advising people to wait, avoid commenting on every Trump tweet, and see what the US actually does.

Lehne, the former EU diplomat, said that there might be a shift in tone coming. He pointed out that it's still so early in the Trump administration that the President doesn't yet have in place a full Cabinet that might reflect broader and less ideological views.

Leone said he was "quite sure" that former ExxonMobil CEO Rex Tillerson, the nominee for secretary of state, "has a different view of international cooperation than Mr. Trump himself. Trump will have to work with his Cabinet, Lehne said, "and he'll have to listen to them to some extent. What we've seen in this week is probably misleading because it expresses the views of a very small circle around him and not reflective of the larger group."

One reality check, analysts and diplomats said, might be the EU's strength as a trade bloc. It represents a market that is currently larger than the US and without the UK, will only be slightly smaller. Lehne said the economic underpinning of the EU makes it more resilient than some people realize, as transnational supply lines and free movement act as a powerful unifier.

And it may bring the US business community into the conversation on behalf of the EU, Burwell said.

"If you talk to US companies, the idea that the EU might break apart and you may have to deal with 28 different countries -- there's no way," she said. "If you don't think the EU is important, just ask these tech companies that look at Europe as a super regulator on issues they care about like privacy."

If Trump tries to make bilateral trade deals with member states, he'll run into a legal roadblock, said one diplomat, because trade negotiations have to be done through the EU capital in Brussels.

Going forward, Lehne said he thinks Trump may simply try to avoid dealing much with EU leaders like Tusk. "He's clearly going to talk to the capitals of the bigger states and if he runs into difficulties will try to play one off the other," Lehne said.