Washington (CNN) European leaders were on edge ahead of this week's potentially confrontational NATO summit in Brussels, Belgium, amid fears that President Donald Trump will follow through on his threat to pull US military protection for allies despite reassurances from officials like Defense Secretary James Mattis.

One senior European official told CNN that NATO members are preparing for a worst-case scenario at the summit, which begins Wednesday, should Trump repeat his threat to end or curtail defense cooperation with NATO allies that are not on a path to hit their defense funding target of 2% of gross domestic product by 2024.

That step would fracture the notion of collective defense that is central to NATO's identity and, Europeans worry, delight Moscow ahead of Trump's meeting with President Vladimir Putin in Helsinki, Finland.

Despite a solid slate of agreements that would, in other times, point to a healthy and thriving alliance, Trump has so undermined confidence that officials on both sides of the Atlantic are bracing for the worst and, at best, hoping for no news at all.

Anger about spending

US and allied officials have already reached agreements that will be formally announced at the summit, including:

* Steps to improve NATO readiness and capabilities to get troops more quickly to any fight.

* An increased NATO role in the training and assist mission in Iraq.

* Enhanced capabilities in cybersecurity.

* The announcement of two new NATO commands, one in the US and one in Germany.

* A continued commitment to increase spending.

But Trump's anger about spending -- which officials say he fails to represent accurately -- is overshadowing those accomplishments. While some diplomats say they are concerned Trump could condition US help on the 2% spending threshold, others worry the President could scale back a planned funding increase for a European deterrence program.

Other European concerns include the possibility that Trump will say something related to NATO military exercises, US troops in Europe or future NATO expansion that could undermine the alliance, according to several officials.

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"The best-case scenario is that it's just rhetoric," the European official said of Trump's comments about a US willingness to maintain its commitment to Europe. "But it is possible that there are consequences, and that is something we're ready for."

The possibility Trump could scale back the defense of countries that fall short of 2% "would really be a threat and that would be new. That's probably the worst-case scenario right now," the official said.

Trump has repeatedly bashed NATO members over the issue of burden sharing since the 2016 campaign and ramped up his attacks in the weeks leading up to the summit.

"Many countries in NATO, which we are expected to defend, are not only short of their current commitment of 2% (which is low), but are also delinquent for many years in payments that have not been made. Will they reimburse the U.S.?" Trump tweeted on Tuesday en route to Brussels.

According to Derek Chollet, a former Department of Defense official and senior adviser for security and defense policy at the German Marshall Fund, "Trump misunderstands what NATO is all about -- he still talks about it as though it's some club in which alliance members owe dues ... they're falling behind on."

"This is money that they spend on their own defense," Chollet said, adding that "part of the problem is that Trump has been uninvolved and uninterested in the details."

Trump has sent a series of letters to NATO allies , including Germany, Belgium, Canada and others, demanding they boost spending and threatening to shift the US military presence in Europe if they do not. The letters were first reported in The New York Times.

The European official noted that top US administration officials are trying to do "damage control" and reassure nervous allies, but in the end "nobody knows what Trump will do."

Indeed, the steady drumbeat of negative comments from Trump is beginning to overwhelm reassurances from the rest of the US national security establishment, said James M. Goldgeier, a visiting senior fellow at the Council on Foreign Relations.

"Officials like Mattis and US Ambassador to NATO Kay Bailey Hutchison have tried to reassure NATO allies, but I think that reassurance is wearing thin," Goldgeier told reporters on Tuesday.

"The Europeans are really bracing for the worst," he said.

One fear is that Trump could buck the advice of key advisers like Mattis and threaten to eliminate a planned funding increase for the European Deterrence Initiative, which is expected to rise from $4.8 billion to $6.5 billion next year, according to Stephen Sestanovich, the George F. Kennan Senior Fellow for Russian and Eurasian Studies at the Council on Foreign Relations.

'Moderate Dog' Mattis

Sestanovich noted that such a move would be consistent with Trump's pre-summit tweets and with recent indications of a possible rift between the President and his defense secretary, a former NATO supreme allied commander. Sestanovich shared a joke going around the Pentagon that Trump is tweaking Mattis' "Mad Dog" nickname and now calls the secretary "Moderate Dog Mattis."

Even if Trump does not make specific threats while in Brussels, some experts argue that the appearance of discord among the allies could undermine the alliance in ways that benefit Moscow. Trump will meet with Putin in Helsinki after the NATO summit and a visit to the UK.

"It's more a concern that the signal coming out of the summit will be disunity. One of Vladimir Putin's core strategic goals is to have the US and Europe divided from each other and that the NATO alliance is weak," Chollet said.

"If we have a summit that is anything like the G7, where a US President is insulting fellow leaders, is refusing to sign an agreement at the end, that would play directly into Putin's hands," he said, referencing the displays of tension between Trump and allied leaders over trade when they gathered in Canada last month.

"What he wants to accomplish is for everyone to have doubts that the US is with them. The stakes are quite high here," he said.

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A senior NATO official downplayed concerns over the future of the alliance.

"We've had disagreements before, we've heard questions about the point of NATO before, and every time NATO has emerged stronger, because it's in the interest of both America and Europe," the official said.

A second European official suggested that Trump's rhetoric is for domestic political consumption and that so far, despite it, the administration is still demonstrating a commitment to NATO.

This official pointed to the two new NATO commands that will be formally agreed to at the summit. One, on the US East Coast, will protect transatlantic sea lanes and another, in Germany, will run logistics to make sure the alliance can respond quickly to threats.

James Carafano of the conservative-leaning Heritage Foundation said the difference in messaging between Trump and top US officials is part of a coordinated effort meant to simultaneously pressure the Europeans. And he said he believes Trump has no intention of "blowing up" the NATO summit -- in part because of domestic concerns.

Any decision to limit US involvement in NATO military exercises could create hurdles in Congress on other issues, like the upcoming Supreme Court confirmation process, Carafano said.

"There is massive bipartisan support for NATO and anger at Russia in Congress. ... If he (Trump) tries to do anything like end exercises ... that would give him a massive black eye when he's trying to get a confirmation through," he told CNN.

Carafano, who was part of Trump's transition team at the State Department, said concerns about what Trump might say at the NATO summit and during his meeting with Putin are "overhyped."

'Hoping to avert disaster'

But other experts warn that Trump's rhetoric could have a lasting negative impact on US relations with some of its most important allies.

Andrew Holland, the director of the American Security Projects' Energy & Infrastructure Studies program, says a US president's rhetoric matters deeply when it comes to foreign policy.

"Part of the game theory of an alliance is signaling ... to rivals and to your partners that you'll stand up ... for your allies," Holland said. Trump's barrage of insults and anger toward Europe "may have signaled to adversaries, to the Russians, that we wouldn't. You have to do the outright signaling."

Chollet, who is headed to Brussels for the summit, said defense officials in the US and Europe "will hope for the best and try not to provoke him."

Trump roiled last year's NATO summit by failing to reaffirm Article 5 commitment to NATO, about mutual defense. "There was a lot of concern about what Trump didn't say," Chollet observed. "Heading into this summit, there's a lot of concern about what Trump will say. Most leaders will be very happy if there's not a lot of news coming out of this summit. They feel good about the substance; they're just hoping to avert disaster."