Washington (CNN) NATO leaders gathering to mark the alliance's 70th anniversary in London face multiple challenges -- including the uncertainty of what President Donald Trump might do.

Trump opened last year's NATO summit with a snarling dismissal of its "delinquent" members for not spending more on their defense budgets and a jibe at Germany for being "a captive of Russia."

This year, Trump is under new pressures -- the 2020 election campaign has begun and he faces an impeachment inquiry that begins its next phase before the House judiciary committee on December 4, the second day of the NATO meeting.

The concern, say NATO watchers, is that the gathering of world leaders will provide an irresistible international stage for the US President to let rip with another series of blistering attacks on NATO members to fire up his base back home.

Fireworks ahead

"The betting is that there are going to be some fireworks," said James Townsend, a former deputy assistant secretary of defense for Europe and NATO.

NATO is working to manage the potential for fallout, scaling back from a "summit" to a "leaders meeting," a nuance that allows the alliance to avoid issuing a formal communique that could cause embarrassment if Trump balks at signing.

And a new cost sharing arrangement established last week is intended to placate the US President and give him something to crow about before he even arrives, analysts say.

"The fear is what he might do," said Townsend, now an adjunct senior fellow at the Center for a New American Security, referring to Trump. "They didn't want to give Trump a platform to blow things up."

Underlying the stage management concerns are deeper questions about NATO's mission. The alliance is being tested by new challenges, even as French President Emmanuel Macron is charging that it is suffering "brain death" and can no longer rely on the US under Trump.

The "allies are approaching the London summit with a sense of foreboding," wrote Karen Donfried, president of the German Marshall Fund. "Few anticipate a gathering that will both unify and stop the growing cracks in cohesion. Alliance leaders carry the responsibility to articulate NATO's common purpose and ongoing relevance."

There are pitfalls if they do not, Donfried said, pointing to Russia. If NATO leaders fail to establish a cohesive vision, Russian President "Vladimir Putin will be raising a glass in Moscow to the fraught state of the alliance at 70."

If Trump and his demands for greater NATO spending created stress on the alliance at last year's meeting, multiple challenges have emerged in recent weeks that will likely test alliance unity this year.

Beyond the questions of how to deal with Russia and China, and how to handle with Afghanistan, NATO is confronted by a challenge presented by Turkey.

The increasingly undemocratic NATO member has cozied up to Moscow, bought Russian military equipment and conducted a widely condemned incursion into Syria that has led some US officials to charge that war crimes are being committed in Turkey's name.

'Brain death'

A US defense official tells CNN that Turkey has now threatened to block proposed NATO initiatives to help defend the easternmost members of the alliance such as Poland and the Baltic States, countries particularly concerned about Russian aggression, unless NATO offers more rhetorical support for its fight against Syrian Kurdish groups that fought with the US and other NATO allies against ISIS.

Asked about the reported Turkish efforts, NATO spokesperson Oana Lungescu told CNN that "NATO's commitment to the safety and security of all allies is unwavering. Our collective defense clause - Article 5 - is at the heart of our Alliance: an attack on one ally would be an attack on all allies."

Macron cited Turkey's incursion into Syria as the justification for his comments that alliance members were experiencing "the brain death of NATO."

Trump opened the way for Turkey to attack by suddenly pulling back troops after a phone call with Turkish President Recep Tayyip Erdogan, but without warning other NATO allies.

"You have an uncoordinated aggressive action by another NATO ally, Turkey, in an area where our interests are at stake," Macron told the Economist shortly after Ankara launched its operation. "There has been no NATO planning, nor any coordination. There hasn't even been any NATO deconfliction ... strategically and politically, we need to recognize that we have a problem."

It's unclear whether Turkey's demands will derail the summit in London, as Ankara is likely to get little buy-in for its Syria operation or its effort to have America and France's Kurdish allies labeled terrorists by NATO. Turkey's incursion has displaced thousands of Kurds and been described as ethnic cleansing by some senior US officials.

But Turkey's push could create tensions. Trump has praised Turkey's role in NATO and took a swipe at Macron's comments during a visit by Erdogan, saying the Turkish president "was very disappointed in the statement made by France having to do with commitment and NATO" and that "many other people feel the same way."

For his part, Erdogan said Macron's comments were "unacceptable."

High level bickering

With the stage set for high-level bickering, analysts point out that Trump might not be the only leader to see the meeting as a platform. Macron may see an opportunity to double down on his comments and provoke a hard conversation about NATO's future.

UK Prime Minister Boris Johnson, who shares some of Trump's populist DNA, is also facing an election and might see an opportunity to juice up his base. And German Chancellor Angela Merkel might yield some domestic benefits back home if she was seen to rein in Macron.

Trump is set to meet with Macron on Tuesday and Merkel on Wednesday.

There are "a lot of politics around these meetings whenever they happen, particularly now," said one British official, speaking on condition of anonymity about the upcoming meeting.

This official said that there could be discussion about Turkey's purchase of the Russian S-400 system and about Macron's criticisms. But the British official, who briefed reporters on the planned meeting, said alliance members felt they were in a "good place" on burden sharing, Trump's fixation.

Trump has long slammed NATO allies, particularly Germany, for not meeting the NATO defense spending target that suggests each member spend 2% of GDP on defense. Only nine of the 29 NATO members currently reach the target. While all NATO allies have pledged to reach the 2% level by 2024 -- part of a push that began under the Obama administration -- not all of them currently have plans to do so.

Secretary of State Mike Pompeo said in a Monday interview with Fox & Friends that NATO members have increased defense spending by $130 billion "so far" and that several hundred million more in increased spending is expected over the next three to four years.

And an agreement reached last week is meant to ease any pressure from the US leader. Trump himself said told reporters Monday at the White House that other NATO countries are "going to do a little more burden sharing."

The US has moved to substantially cut its contribution to NATO's collective budget -- the pot of money used to cover the cost of maintaining NATO headquarters, joint security investments and some combined military operations -- while other nations have increased theirs, according to several US and NATO officials.

'A Kabuki dance'

Previously, the US provided some 22% of NATO's direct funding, based on an agreed-upon cost-sharing formula based on each member's gross national income.

The officials tell CNN that the Trump administration sought to lower its contribution to about 16%, in line with Germany, which provides 14.8%, even though the US has a much larger economy.

US and NATO officials told CNN that the other NATO members will make up the shortfall.

"Cost shares attributed to most European allies and Canada will go up, while the US share will come down," a NATO official told CNN.

The total amount is small enough that it won't make much difference, said Townsend. In 2019, about $260.5 million was set aside, mainly to fund the NATO headquarters in Belgium and its administration.

A military budget of $1.56 billion funds some joint operations, the NATO strategic command center, as well as training and research. And a joint budget for the NATO Security Investment Programme, which covers major construction and command and control system investments, is capped at $770 million for 2019.

"It's a kabuki dance ... so Germans can say they're doing more, the US can say they're doing less," said Townsend. The problem that is has set off "a row in the alliance," particularly among countries such as France, which is the only country so far refusing to pay more.

And that, said Townsend, could give Trump an opening to exploit. "Who knows how this is going to play, but it has caused problems and with Trump, a lot of his response is of the moment, maybe he will use that as a pretense to fly off the handle."