President Donald Trump sent shock waves across Europe when he railed against the NATO alliance during his campaign and threatened not to defend those nations that hadn’t paid their share of defense expenses.

He wasn’t the first to raise concerns: President Barack Obama had frequently lamented NATO members’ failure to spend enough on defense.

But it was Trump’s undiplomatic rhetoric that got the issue to the top of the group’s agenda this week, when NATO’s members are expected to accept the idea of public report cards to make sure everyone’s meeting the requirements of the alliance.

Obama complained three years ago at a gathering of European Union leaders that defense couldn’t be left just to the United States and Great Britain. Citing Russia’s annexation of Ukraine’s Crimea region, Obama said the situation “reminds us that our freedom isn’t free.”

Six years ago, then-Defense Secretary Robert Gates went further, warning NATO leaders that younger U.S. politicians – “those for whom the Cold War was not the formative experience that it was for me” – might abandon the six-decade-old defense alliance if allies didn’t carry a greater portion of the load.

But it was Trump who scared leaders into accepting closer monitoring of their defense spending when he called the 28-member organization “obsolete” and threatened to pull out or significantly cut funding.

As Trump boarded Air Force One on Wednesday on his way to Brussels, NATO Secretary-General Jens Stoltenberg met with reporters here in Brussels and commended the Trump administration for setting an example by presenting a budget that would increase military spending in Europe by 40 percent.

“This is a commitment to our collective defense from the United States, not only in words but also deeds,” he said.

Stoltenberg said he expects leaders to approve a new NATO initiative by which each member would submit detailed annual reports on their progress toward increasing defense spending to support the organization.

“This will be a new tool to ensure we keep up the momentum and live up to our commitments,” Stoltenberg said.

Trump’s arrival in Brussels on Wednesday after a meeting with Pope Francis at the Vatican marked the beginning of the second half of his five-country tour. The first half was billed as a religious pilgrimage that took him from Saudi Arabia to Israel and then to Rome.

The second half will feature a meeting Thursday with NATO members and European leaders to discuss what more NATO allies can do to fight terrorism and to share the alliance’s financial burden.

It has the hallmarks of another awkward setting. Beyond his campaign assertion that NATO was obsolete – he’s since withdrawn that characterization – Trump has come under for fire for allegedly, German officials claimed, handing German Chancellor Angela Merkel a fake “bill” for overdue NATO payments during their meeting in Washington in March. The White House denied he’d done any such thing.

Still, Trump will be under pressure to show that he backs the NATO alliance, something its leaders will want to see given the cloud of controversy in Washington over possible contacts between his campaign team and Russia, an adversary to many in the European Union.

Members are waiting to see whether Trump will deliver a populist message, and repeat comments that angered them when he hailed June’s Brexit vote, which will lead to Britain’s departure from the EU, and and predicted that other nations would follow.

Considering NATO’s efforts to confront Russian aggression in Ukraine, members are particularly concerned that Trump has yet to explicitly endorse Article 5 of NATO, which is the part of the treaty that defines an attack on one member state as an attack on all.

Unlike Vice President Mike Pence and Defense Secretary James Mattis, Trump has never said he’s committed to that core mission. That has some members concerned, said Thomas Wright, the director of the Center on the United States and Europe at the Brookings Institution in Washington.

“He said NATO’s not obsolete anymore, but when he says that, he only says that because it’s fighting terrorism, not because he has endorsed its original mission,” Wright said. “He has yet to endorse its original mission. I think that might be a bit of a problem.”

Speaking with reporters aboard Air Force One, Secretary of State Rex Tillerson said Trump supported Article 5. Asked whether the president will say so explicitly, Tillerson said, “He is still working on final remarks, so I don’t want to tell you exactly what is going to be in the speech.”

Stoltenberg downplayed the controversy Wednesday, saying Trump has pledged strong support for NATO. By doing so, Stoltenberg said, the president is in effect supporting the core missions of NATO.

“So by expressing strong support to NATO, to our security guarantees, the United States, President Trump, has also, of course, expressed strong support to Article 5, because Article 5, collective defense, is NATO’s core task,” he said.