Donald Trump's "America first" approach has Europe worried he may cut US commitments to NATO just as it mounts its biggest military build-up since the end of the Cold War to counter a more assertive Russia.
Trump caused uproar during the campaign when he suggested Washington would think twice about coming to the aid of an endangered NATO ally if it had not paid its dues.
The fear is that Trump embodies an isolationist tradition -- "avoid entangling alliances" -- which will add to uncertainties as Europe faces challenges new and old from the east, the Middle East and North Africa.
"A Trump administration will increase US isolationist tendencies, which is a further blow to (its global) leadership role," said Fabian Zuleeg, chief executive at the European Policy Center in Brussels.
The United States set up NATO to protect post-war Europe from the Soviet Union and its "all for one, one for all" collective defense guarantee has stood the test of time.
But for many years Washington, which accounts for nearly two-thirds of combined NATO defense spending, has demanded that its 27 allies do more to share the burden.
Trump's harking on this issue during the campaign caused such fears in the Baltic states and former Soviet-ruled eastern European NATO members that US Vice President Joe Biden was sent on a reassurance mission.
"Don't listen to that other fellow -- he knows not of what he speaks. America will never fail to defend our allies," Biden told them.
Now that "other fellow" is set to be the 45th President of the United States of America.
NATO in shock
The shock in Europe Wednesday was palpable.
In an unusual series of public statements shortly after Trump's victory was confirmed, NATO head Jens Stoltenberg stressed the continued importance of US global leadership.
"Our alliance has brought together America's closest friends in times of peace and of conflict for almost 70 years. A strong NATO is good for the United States and good for Europe," he said.
Poland's President Andrzej Duda urged Trump to stick by commitments to boost NATO's presence on its eastern flank to ensure allies would not be left in the lurch if Russia attempted another Ukraine-style adventure.
"We sincerely hope that your leadership will open new opportunities for our cooperation based on mutual commitment," Duda said.
The Polish deployment is led by a US battalion and so is especially emblematic.
European Council head Donald Tusk, a former Polish premier, weighed in with a plea that the European Union -- of whose 28 members 22 also belong to NATO -- and the United States work together in defense of shared values.
"I do not believe that any country today can be great in isolation," he added, alluding to Trump's campaign slogan "Make America Great Again."
EU defense opportunity?
For some analysts, the fear factor may be overdone -- foreign policy was not a major issue in the election, Trump's focus is domestic and it is much too early to say what he will actually do as president.
Ian Lesser, senior director with the German Marshall Fund of the United States in Brussels, said the new president "is not so much an isolationist as a rigorous unilateralist ... who may demand a great deal from the allies."
"For him, foreign policy starts with homeland security and works out from there," Lesser said.
A key test will be Russia and whether Trump keeps campaign promises to improve ties strained to breaking point by the Ukraine crisis.
Russian President Vladimir Putin cited those promises when congratulating Trump, insisting: "Russia is ready and wants to restore fully-fledged relations with the US."
Where that would leave Europe, and especially NATO's eastern allies, is a major uncertainty.
But it is perhaps also an opportunity, in the same way that Britain's shock vote to quit the EU has cleared the way for France and Germany to press the sort of increased military cooperation London has resisted for years.
"It is the moment to realize that what is being done (to boost Europe's own defense capabilities) is not entirely useless, that there is a good reason for it," an EU diplomat said.