BRUSSELS, Belgium – As President Trump feuded with some of America's closest allies last week and talked about inviting one of their great rivals, Russia, back into an exclusive economic club, Jim Mattis was here sounding a very different note.

Standing at a lectern inside NATO's gleaming new headquarters — marked by reminders of the transatlantic alliance's powerful history — Trump's defense secretary declared that "the American people remain committed to this alliance, and we look forward to working together."

It was yet another example of Mattis, a former Marine general, reassuring America's closest friends that their bonds remain strong, a familiar and soothing contrast to the combative tone coming from the White House.

It's a role that has made Mattis a critical American voice on the world stage.

"He is, without question, within Europe, and really, I would argue, around the world, the most respected person in President Trump's Cabinet," said Derek Chollet, a former assistant secretary of defense, who managed Europe and NATO policy under two of Mattis' Obama-era predecessors.

There are looming questions, though, over how far Mattis' words can go when contrasted with the president's increasingly bruising approach to allies such as Canada, the United Kingdom, France, and Germany.

"Last year it was more rhetoric and bluster [from Trump], but now we're seeing the rhetoric and the bluster turning into real action," said Julie Smith, a former principal director for European and NATO policy in the Obama-era Department of Defense. "Now Mattis has a harder job."

Three days last week provided a glimpse of how he has tried to balance his roles tending to alliances while also serving a go-it-alone president.

‘A disunifying force’

International strains were growing even as Mattis crossed the Atlantic for a gathering of NATO defense ministers.

The previous week, Trump had imposed tariffs hitting Canada and American allies in Europe — citing national security reasons as he targeted some of the United States' oldest friends. Europe hit back by imposing levies on American products including bourbon, peanut butter and Harley-Davidsons.

The trade fight was only the latest example of months of friction. Frustration in Europe has grown enough that some world leaders, most notably German Chancellor Angela Merkel, have questioned if the old order will still hold.

"The NATO alliance doesn't work if the U.S. isn't actively leading it," said Chollet, a visiting fellow at Penn's Perry World House and executive vice president at the German Marshall Fund of the United States.

American leadership after World War II brought together a fractured Europe and continued under presidents of both parties. But now, Chollet said, "the United States is being seen as a disunifying force within Europe."

Mattis is a former NATO commander who sees great value in international cooperation, and he stressed the importance of alliances as he rumbled east in a Boeing E-4B nicknamed "the Doomsday Plane."

Decades old, the aircraft has boxy consoles and analog instruments — but it's also capable of surviving an electromagnetic pulse from a nuclear strike and serving as a flying command center.

Within the brawn were delicate touches: Air Force crew members had set out cucumber infused water and served blueberry crepes for breakfast.

Speaking to reporters inside his quarters, stocked with old video consoles and a spartan bunk bed, Mattis downplayed the idea that Trump's actions could disrupt key alliances. One or two disagreements, he said, aren't enough to shake shared democratic values and decades of collaboration on security.

"It's almost like you have an orchestra playing, but you got cymbals clashing. We'll address the cymbals, no problem … but that does not refute the fact that the violins are in harmony," said Mattis.

Despite — or perhaps because of — the weight of his job, the 67-year-old gave the impression that he rarely slept. The room had a bunk bed, but all Mattis talked about was the real work he had to do once he was done with the media, despite the short turnaround from his 19-hour flight from Singapore less than three days earlier.

The former general earned a reputation as a fearsome military leader, but also a philosophical one who reads voraciously. He's sometimes called a "warrior monk," though Trump prefers "Mad Dog."

Mattis acknowledged the anxiety he was likely to face in Europe.

"The reason I'm traveling there is to listen, to take notes, to work these things forward," he said.

Trump, tariffs and Twitter

The friction comes from more than just tariffs.

Trump, who once declared that he was elected to represent Pittsburgh, not Paris, has also pulled the U.S. out of pacts with allies on climate change and Iran's nuclear program.

Just as Mattis made the rounds at NATO, the president dove into a public spat — via Twitter — with French President Emmanuel Macron and Canadian Prime Minister Justin Trudeau.

"In Europe, there's extremely high levels of anti-Trump and anti-Americanism in general," said Mitchell Orenstein, a professor of Russian and East European studies at Penn.

He pointed to polls last year from the Pew Research Center that found that even before the recent clashes, most Europeans — approaching 90 percent in France and Germany — lacked confidence in Trump's leadership on the world stage.

Signs of very different eras flank the walkway into NATO's new headquarters.

On one side is a slab of the Berlin Wall. On the other, twisted metal from the North Tower of the World Trade Center. One recalls the decades when transatlantic allies stood against communism. The other marks a moment when Europe came to America's aid.

The meeting of defense ministers Thursday and Friday was the inaugural event at the new site, with its sleek interior of glass, cool grays and white. Their talks took place far from public view, in an area blocked off by security.

When officials met with reporters, though, the tariff issue loomed. European journalists repeatedly pressed NATO Secretary General Jens Stoltenberg about the trade clash.

Trump has questioned the cost of America's deep international involvement, in both money and lives. While previous presidents had urged NATO members to spend more on defense, Trump has turned up the pressure.

Mattis has faithfully carried the president's message on spending, pushing his foreign counterparts to invest more in defense, while also promoting the importance of unity.

Allies may find Mattis' words soothing, said Penn's Orenstein, but his reassurances are undercut by Trump's belligerent tweets and frequent policy swings. Military alliances, he said, depend on predictability and reliability.

"The weird thing about the Trump administration is that he has his own personal foreign policy that is different from the policy of his administration," Orenstein said.

Harmony — and disruption

The U.S. and Europe have been through difficult periods before.

Stoltenberg, the NATO chief, pointed to the split over the Iraq War as the most recent example whenever he was asked about Trump's tariffs.

"It's nothing new that there are differences between NATO allies," he said. "And what we have seen again and again is that we have been able to unite around NATO's core task, to protect and defend each other despite those differences."

And when it comes to NATO, Trump's actions line up with what previous presidents did, Chollet said. The U.S. is beefing up European defenses and will host a new NATO headquarters in Norfolk, Va., monitoring Atlantic waters where Russia has tried intimidation tactics — even as Trump continues to shower affection on the rival country.

Most of Mattis' work here was done in private meetings with fellow defense ministers. Amid the chilly Trump-Merkel relationship, Mattis sat with Germany's defense minister — though there were few details provided about their discussion.

When Mattis did emerge for a news conference, an awkward scene played out: a TV crew had an audio problem, leaving the defense secretary unhappily fidgeting.

For days Department of Defense aides had warned reporters to arrive at events 20 minutes ahead of schedule, knowing Mattis' habit of coming early and leaving fast. If the sound didn't get fixed quick, an aide said, the general would have to leave.

"I can't tell you how much that would break my heart," Mattis said. "I need to go off and actually work for a living."

The sound problem solved, Mattis touted a 2017 surge in NATO members' defense spending — the largest increase in 25 years, he said — and steps the alliance was taking to confront terrorism and an aggressive Russia.

But back in the U.S., Trump was about to leave for his own meeting with world leaders, including Macron, Merkel and Teresa May, prime minister of the United Kingdom. As he did, the president called for Russia to be invited back into their club of the globe's largest economies — defying the allies and his own generals, who have stressed efforts to deter Moscow, not reward it.

It was another disruptive burst that again clouded the pledges of harmony in Brussels.

And it came as his defense secretary was back aboard the plane, this time heading to London, to sit with leaders from another ally grappling with America's confounding new posture.