Yet, just days away from the White House, the President-elect is sparking transatlantic turbulence by expressing fresh disdain for the two institutions that have been the bedrock of post-World War II peace, stability and prosperity -- NATO and the European Union.
It is too early to tell how the Trump administration will shift America's stance toward Europe, since details have been at a premium both during the election campaign and the transition when policy often has been announced 140 characters on Twitter.
But European governments worried about Trump's arrival in the White House now have new reasons to be fearful.
Fresh comments by Trump in an interview with two European journalists raise the extraordinary scenario of a new American president who seems to question the fundamental purpose of Western security infrastructure -- an unthinkable prospect ever since the defeat of Nazism and throughout the Cold War.
The President-elect warned that NATO was "obsolete," criticized German Chancellor Angela Merkel's policy of welcoming Middle Eastern refugees as a mistake, and predicted the United Kingdom would not be the last nation to leave the EU.
Although he said Europe could still count on US security guarantees and that he felt "very strongly" toward the continent, Trump's comments on NATO were a stunning departure for someone who will be President in a few days.
"No one in mainstream political life has ever questioned NATO and US membership in it," said Jamie Kirchick, a journalist who specializes in foreign policy and the author of "The End of Europe: Dictators, Demagogues and the Coming Dark Age," to be published in March.
While other US presidents and officials have long vented frustration at the failure of all NATO members to meet their funding commitments, "never have you had someone come out and say NATO is obsolete," Kirchick said.
Former US ambassador to NATO Ivo Daalder expressed the sense of unpredictability in a tweet, warning: "Trump is more critical of NATO, EU, & Germany -- all close allies -- than he's ever been of Putin & Russia. We're entering an upside down world."
Trump argued in a press conference last week that it would be an "asset" to America if he could forge a good relationship with Russian President Vladimir Putin, defending his openness to dialogue with a leader who many of his own fellow Republicans view as an unrepentant enemy of the United States.
Trump's new salvo means Europe's leaders can no longer console themselves with the thought that Trump's contempt for the transatlantic establishment and tolerance for the world view of Russian President Vladimir Putin can be easily dismissed as the fungible rhetoric of a populist campaigner that will disappear once he's confronted with the responsibilities of being President.
Trump will be President in days
As commander-in-chief, Trump will be in a position to emulate -- on an even wider, global, stage with higher stakes -- the disruption and convention-busting style that shook the Washington establishment.
That could mean a rocky time is beckoning for US allies as he questions the unchallenged assumptions that have underlined US power in the world for decades.
At the very least, Trump's blast toward Europe is causing confusion about the future course of US foreign policy, given that it differs sharply from the strong pro-NATO, anti-Russia line stated by several of his cabinet nominees, including defense secretary designate James Mattis, last week.
In a joint interview with the German newspaper Bild and the Times of London, Trump renewed his contention that NATO is outdated because it has failed to fight terrorism, even though the alliance has been fighting to prevent Afghanistan becoming a terror haven again for years since 9/11.
Tearing at fragile European divisions, he described the EU as a "vehicle" for German control, and praised Britain for voting to leave -- breaking with decades of US foreign policy orthodoxy that sees a strong, unified Europe as vital to global security.
The President-elect also said he had equal trust for Merkel, the undisputed first among equals of European leaders and strong supporter of the transatlantic alliance, and Putin, who US intelligence agencies have accused of interfering in the election.
And shredding taboos about intervening in the campaign politics of a foreign nation, Trump caused Merkel a political headache in a tough re-election year by saying that though he respected and admired her, the German leader's decision to welcome hundreds of thousands of Middle East refugees was "catastrophic."
German Foreign Minister Frank Walter Steinmeier said Trump's comments caused "astonishment and agitation" in Europe. "I have just had a conversation with the NATO Secretary General Jens Stoltenberg, who is concerned that President-elect Trump regards NATO as obsolete," Steinmeier said.
Merkel's office was more tempered, pledging to work closely with the new US president.
But other European officials rejected Trump's view that the EU was about to fall apart and they took the line that once it takes power soon, the Trump administration will undaerstand the stakes in Europe and adjust policy accordingly.
"For the last 70 years, Europe and the US have always agreed on one thing -- and that is European unity is in the interests of both parties, both the US and Europe," Frans Timmermans, vice president of the European Commission, told CNN's Christiane Amanpour.
"We have built from scratch something that is arguably the most successful peace project in human history," he added. "I am sure that sooner or later, everyone in Washington will understand that it is in the strategic interests of the United States to have a successful European Union."
Trump's rhetoric or is it reality?
Trump's comments on NATO are consistent with the campaign rhetoric in which he expressed a transactional role of US global alliances, complaining that Washington's partners are not paying enough for defense.
But they are also a startling departure from seven decades of US foreign policy orthodoxy that has used NATO and alliances in Asia to project US power in defense of a liberal, democratic world order.
Given all this, Trump's inauguration on Friday will open a period of uncertainty for US allies.
"We have never had a President-elect quite like Mr. Trump," former US Middle East peace negotiator Aaron David Miller told CNN on Monday, heralding a new and unpredictable phase dawning in America's relationship with Europe and the world.
"In four days, the ultimate pivot is coming, when, in fact, Mr. Trump will bear responsibility for existing and future US foreign policy."
NATO nations will have to decide whether they believe Mattis that NATO is a "vital" US national security interest or take Trump's comments at face value. Some European governments had been assured by indications earlier in the transition that Trump was backing away from his campaign rhetoric. For instance, in a call with British Prime Minister Theresa May in December, a Downing Street statement said the two leaders agreed "on the importance of the Alliance."
But Trump's new comments to the Times and Bild could undermine those hopes.
One possible explanation for the President-elect's anti-NATO rhetoric is that, as in many other areas -- for instance China policy, where he has tweaked Beijing over Taiwan -- Trump may be adopting hardline positions that are meant to shock as an opening negotiating position. He's often said he wants to be unpredictable.
"In this case, it is impossible to know, is this an effort on the part of the President-elect to be predictable, to keep allies and adversaries off balance?" asked Miller.
In NATO's case, Trump's aim could be to cajole more members to reach the 2% threshold of GDP target on defense spending required by the alliance.
"One thing's for sure, European nations will have to take more responsibility for their own security," said Timmermans. "I'm a strong believer in NATO, it's indispensable. The only one who wants to see NATO fall is Putin."
But Trump's approach and determination to improve strained US relations with Russia can only add to the deep uncertainty stalking Europe that is already strained by Britain's vote to leave the EU and critical elections looming in Germany and France, in which populist anti-establishment forces are on the rise, along with concerns about the specter of a rambunctious Russia to the east.
Though NATO is a 28-member alliance run by a secretary general, it always takes its lead from the US, its most powerful and wealthy member. That makes for unprecedented times, with a US President who appears to doubt the alliance's role.