BRUSSELS — Secretary of State Mike Pompeo attended meetings at NATO on Wednesday amid questions about U.S. leadership in the military alliance and whether it will even continue to be a member.

Pompeo’s conversations with foreign ministers at the headquarters of the North Atlantic Treaty Organization were meant to lay the groundwork for a higher-level meeting of presidents and prime ministers scheduled for next month in London.

Once pretty staid affairs, NATO gatherings nowadays are infused with melodrama and cliffhanging uncertainty about global challenges and the willingness of the United States to aid its allies. Diplomats are particularly concerned about the U.S. decision last month to withdraw troops from northern Syria before a Turkish invasion, without consulting the Europeans in advance.

Testimony by State Department officials at the impeachment hearings in Washington has also made some of Pompeo’s own diplomats question his strength of purpose, while a growing number of European diplomats say they believe he ultimately yields to President Trump’s wishes even if he considers them unwise.

Televisions at NATO headquarters showed live coverage of the impeachment inquiry, which focuses on whether Trump used his office to try to force Ukraine to take actions damaging to his political opponents. Pompeo repeatedly rebuffed questions about the proceeding and an assertion by Gordon Sondland, the U.S. ambassador to the European Union, that the secretary was briefed on the effort every step of the way.

Asked at a news conference whether he had watched any of the proceedings, Pompeo grew testy. “I didn’t see a single thing all day,” he said. “I was working. Sounds like you might not have been.”

When later asked specifically about Sondland and whether he should recuse himself, Pompeo replied: “I’m not going to recuse myself from this. I know precisely what American policy was with respect to Ukraine. I was working on it, and I’m very proud of what we’ve accomplished.”

Concern about the use of NATO is nothing new, but criticisms about the alliance that were once largely expressed behind the scenes are being aired more openly as the London summit approaches. Leaders are steeling themselves for another public thrashing from Trump, who suggested at last year’s summit that the United States may withdraw from the alliance if European governments don’t spend more on defense.

French President Emmanuel Macron told the Economist magazine that NATO may become obsolete — “what we are currently experiencing is the brain death of NATO,” he said — because the United States seems to be “turning its back on us.”

Pompeo sought to soothe the worries during a visit to Germany early this month, extolling NATO as “one of the most critical, strategic partnerships in all of recorded history.”

Only France has publicly aired its complaints. But Ivo Daalder, president of the Chicago Council on Global Affairs and a former U.S. ambassador to NATO, said several European officials have told him they fear the United States will not live up to its commitment to mutual defense embedded in Article 5, invoked only once in history, after the 2001 terrorist attacks on the United States.

“You don’t want to ask questions about fidelity if you want to stick in the marriage,” Daalder said. “Other countries say they’re worried, but they don’t ask because no one wants to know the answer.”

The draft schedule for the London summit suggests organizers are trying to limit the amount of time leaders spend in each other’s company. There will be a reception with the queen, but no formal dinner afterward, and a working session at a luxury hotel and golf course outside London.

“NATO summits used to be fairly boring, formulaic processes,” said Heather Conley, director of the Europe program at the Center for Strategic and International Studies. “Since the president openly questioned U.S. engagement with NATO, we approach all these leaders’ meetings with great trepidation — got to lock things down, get declarations, limit the damage. The process has been turned on its head.”

Among the top security questions before the alliance are the incursion by Turkey, a NATO member, into areas under Kurdish control in northern Syria, and why Europeans didn’t hear about it until after the fact.

The U.S. decision to remove troops and support for Kurdish fighters in the area underscored the erosion of an implicit understanding among NATO allies that they be transparent, reliable and forthcoming with one another, usually behind closed doors, said Douglas Lute, a former U.S. ambassador to NATO.

“This meeting comes at a real intersection for NATO,” he said. “That’s because NATO is facing a whole series of challenges, internal to the alliance and external. On top of that, they’re having to contend with the challenges at a time when U.S. presidential leadership is more questioned than at any other time in the history of the alliance.”

In greeting Pompeo at NATO, Secretary General Jens Stoltenberg said they would address strategic issues like Russia, arms control and the implications of the rise of China. “NATO remains the only the platform where North America and Europe sit together, meet together and make decisions together of great importance,” he said, thanking Pompeo for his personal commitment to NATO “and the way you help to move this alliance forward.”

Pompeo predicted they would have a successful round of meetings “so we can have a successful leaders’ summit and continue to drive this incredibly important NATO mission forward.”

But skepticism runs deep, of the United States and Pompeo.

“People discount what Pompeo says because they know he doesn’t push it with Trump,” said Tom Wright, director of the Center on the United States and Europe at the Brookings Institution. “I’m sure he’ll make a supportive statement. But it may not be enough.”

Daalder said Pompeo should make the point that NATO operates normally on the day-to-day level, working to keep Americans and Europeans safe.

“He should make the case that NATO is important to the United States,” he said. “Remind people, NATO is not something that comes to life one day a year. And hope the president is in a good mood when he comes to London.”