"We know the direction: less US in the alliance," said one NATO diplomat, talking like others on condition of anonymity to discuss sensitive subjects.
"Our interest is to keep the maximum US in the alliance."
So far, US officials have insisted that Washington remains committed to an alliance that has seen American military might underpin Western security for over seven decades.
That's at least some reassurance after Trump reportedly threatened to pull the United States out entirely during his first term.
Still, the warning signs are flashing red.
Those range from the deep contempt for Europe voiced by Trump's team, to territorial threats against Greenland and Canada, and the president's threats to only protect allies spending enough that strike at the heart of NATO's mutual defence pledge.
Meanwhile, as Washington makes clear it could shift forces away from Europe to focus on other challenges, Trump's outreach to Russia has unleashed fears he could end up siding with number one nemesis Moscow.
"The level of optimism is certainly dropping," a second NATO diplomat told AFP.
"The US still hasn't taken any concrete decisions, but every day seems to bring another hit against the foundations of the alliance."
- Withdrawing from Europe? -
US defence chief Pete Hegseth fired the first shots in February, warning European allies in a fiery debut at NATO that they would have to "take ownership of conventional security on the continent", with Washington "prioritising deterring war with China in the Pacific".
Trump's is not the first US administration to float pulling back from Europe: Barack Obama vowed a "pivot to Asia" in 2011.
"We always knew that the moment would come when America would sort of step back and Europe would have to do more -- and that's not a bad thing," said Jamie Shea, a former NATO official now with the Chatham House think tank.
AFP
US President Donald Trump's administration has warned it will no longer be able to be primarily focused on Europe
"The idea is to try to keep as much American engagement as you possibly can, and if they do start withdrawing, at least trying to make sure that they withdraw the less important stuff and the door is always open for them to ramp up again."Diplomats and officials insist that if the United States is planning to scale down its presence in Europe then it needs to lay out a clear timeline so as not to leave gaps in the continent's defence.
Europe is ramping up its expenditure and weapons production to be ready, spurred on by Trump's demand to more than double NATO's spending target to five percent of GDP.
Even then, officials concede that replacing key assets like US nuclear capabilities will prove almost impossible.
"The Americans have got to be honest with the Europeans about what they're going to take away and what they're going to leave -- because at the moment, the problem for Europe is it has no idea," Shea said.
"With Trump's performative politics, strategic thinking and long-term planning come much behind short-term punitive measures and headlines, and that means all this could be severely bungled."
- Siding with Russia? -
While US involvement inside the alliance may be one issue, NATO members in Europe are also fretting about potentially more fundamental threats.
NATO is essentially based on an act of faith -- article five of its founding treaty that declares an attack on one ally "shall be considered an attack against them all".
That provides succour to allies, and wards off adversaries, provided everyone believes it. But Trump has cast doubt on the US commitment, suggesting he would only protect countries spending enough.
"Article five is about psychology -- and in this sense he did question it," admitted one diplomat.
Beyond that, there is another looming unthinkable haunting the alliance.
As he reaches out to Russia on Ukraine and sidelines Europe, could Trump genuinely be willing to draw close to their prime enemy Russian President Vladimir Putin?
For now the hope is that Trump's courting of Moscow is part of his negotiating strategy to try to secure a deal to end the war.
"It can be a bitter medicine to swallow but, if it's just tactical, of course it could be acceptable," a fourth diplomat said.
"If it is a strategic shift, then of course it's a big problem."
