The US president told a Montana campaign rally last week he's coming to "tell NATO: 'You've got to start paying your bills. The United States is not going to take care of everything!'" The crowd erupted in applause as Trump returned to his familiar rant against Germany's low defense spending and its leader, Chancellor Angela Merkel. He said he'd "told Angela" that he "can't guarantee it, but we're protecting you and ... I don't know how much protection we get protecting you."   

This chipping away at the bedrock of the alliance — the mutual defense pledge embodied in its Article V — is just the latest threat to NATO's hopes to pull off a polished well-choreographed summit on June 11 and 12 to christen its glossy new headquarters. Now a "success" may well be characterized as the absence of diplomatic disasters like those that befell the G7 in early June, where Trump refused to sign the final communique, called Canadian Prime Minister Justin Trudeau "dishonest" and was captured in a photo that went viral glaring across the table at an equally stern German chancellor.

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"We're absolutely right to be nervous," said Sophia Besch, a fellow with the Centre for European Reform (CER), about the possibility a Trump-Merkel clash could derail the summit. "[Trump] is framing the 2 percent as the golden ticket for any European leaders to get into his good graces, and that's why everyone is so worried." Only seven allies other than the US invest in their militaries at the desired level, though all 29 are increasing their defense budgets now, a trend that started in 2014.

'Digs at NATO serve Putin'

While he wrote letters to several allies warning them their spending levels were not satisfactory, Trump's antagonism is expected to be largely directed at Merkel for managing Europe's largest economy but only allocating 1.2 percent of GDP to defense this year, with a plan to increase to 1.5 percent by 2025.

While agreeing Germany needs to upgrade its defense more robustly, Besch feels the relentless Trump rhetoric is counterproductive and even dangerous. "Everything that undermines the trans-Atlantic alliance works in [Russian President Vladimir] Putin's favor at the moment," she told DW. "The way that President Trump singles out Angela Merkel potentially divides Europeans as well." Besch urged allies not to be "distracted" by the funding flap and risk losing sight of other priorities.

Agitation on the agenda

But James Appathurai, NATO deputy assistant secretary general for political affairs and security policy, explained the alliance can't short-circuit the discussion about "cash, capabilities and contributions."

"It is very important that we don't end up in a situation where the United States gets so frustrated that it does start to feel like it needs to take other decisions," he explained. "That would be a terrible outcome for everyone. So what we need to do is demonstrate that burdens are being shared fairly." Trump has already suggested he may pull out some of the 35,000 US military personnel currently stationed in Germany.

There are plenty of other issues which do require attention, a unified response to Russian aggression being one of the most obvious. New concerns have arisen from Trump's non-committal response to a question about Russia's annexation of Ukraine's Crimea region, the single most transformative event in NATO's post-Cold War history. Asked in the context of his planned meeting with Putin on July 16, Trump said: "We're going to have to see" whether he'll change US policy on non-recognition of the land grab.

Efforts to end the ongoing war between Ukrainian government forces and Russian-backed separatists in eastern Ukraine are also languishing. Deputy chief monitor of the Organization for Security and Cooperation in Europe mission Alexander Hug urged leaders at the summit to push the two sides, along with Moscow, to implement their ceasefire agreement. "No chance of discussing the conflict should be missed," Hug told DW. "It would be tragic if at the platform where security is a big part of the standard agenda, the conflict in and around Ukraine is not being discussed."

Read more: Germans protest US military movements outside Berlin

All's well that ends ... with a communique

Ian Lesser, vice president for foreign policy at the German Marshall Fund in Brussels, said the summit could prove to be a productive one, if nothing goes wrong, with "some very concrete deliverables on everything from burden sharing to rapid response to partnerships on the European periphery." But, he noted, after the G7 experience, officials must now worry there might not be a final communique at all.

"If it was simply about the longer-term issues of adjustment over burden sharing or NATO's role in counterterrorism and things of that kind, I think it would be a different discussion," Lesser explained. "But there is a great sense of frustration that somehow the traditional moorings of trans-Atlantic diplomacy, including trans-Atlantic diplomacy at NATO, have been lost, that somehow we've come adrift from the normal predictable ways of doing things and perhaps in part that's what the administration is trying to encourage."