NATO Secretary General Jens Stoltenberg drew bipartisan applause and some 18 standing ovations Wednesday as he made a case to Congress for the survival of the transatlantic alliance that was built out of the ashes of World War II, but his most important audience was not in the room.

Addressing a joint meeting of the House and Senate to commemorate NATO’s founding in Washington 70 years ago this week, Stoltenberg aimed much of his approximately 40-minute speech at answering President Trump’s skepticism and occasional hostility toward the alliance, while throwing in some praise of the president as well.

The NATO chief thanked the United States for building and sustaining the alliance as it grew from 12 members to 29 and credited Trump with forcing a reckoning among NATO nations over how they fund their joint commitment to military defense.

“NATO allies must spend more on defense. This has been the clear message from President Trump, and this message is having a real impact,” Stoltenberg said, prompting Vice President Pence, seated behind him, to leap to his feet. House Speaker Nancy Pelosi (D-Calif.) followed a few moments later.

The NATO chief spoke at the invitation of Pelosi and Senate Majority Leader Mitch McConnell (R-Ky). The rare joint invitation is a mark of Republican distaste for Trump’s harsh treatment of NATO allies.

Trump did not invite other NATO leaders for a grand summit to honor the anniversary, and many of those leaders were reluctant to hold such a meeting anyway because of Trump’s confrontational behavior at past gatherings.

“We have to be frank,” Stoltenberg said. “Questions are being asked on both sides of the Atlantic about the strength” of NATO.

That was an oblique reference to open speculation among allies about whether Trump’s standoff with traditional partners such as Germany poses an existential threat to the alliance.

Stoltenberg also nodded at other criticisms of the alliance that go beyond those raised by Trump.

Apparently addressing European pacifist distrust of defense spending, Stoltenberg said that “peaceful protest” and “dialogue” have not stopped aggressors from Adolf Hitler to Joseph Stalin to the Islamic State.

“We must invest and act,” he said, prompting one standing ovation.

Stoltenberg drew bipartisan applause for lines noting that NATO was founded as a counter to Soviet aggression and that it remains a counter to an aggressive and unpredictable Russia. In addition to conflicts in Georgia and Ukraine, Russia is responsible for “attempts to interfere in democracy itself,” Stoltenberg said.

Republicans including McConnell have broken with Trump over his overtures to Russian President Vladi­mir Putin and willingness to accept Putin’s denial of interference in the 2016 U.S. presidential election.

Many Republicans are also uneasy about Trump’s equivocations on NATO obligations, and Stoltenberg drew a standing ovation for his note that the alliance is based on the premise of “all for one and one for all.”

Trump has never swayed from his campaign-trail view that European NATO allies ride free on the back of American money and military power while taking advantage of the United States in trade arrangements.

“The United States alone accounts for the vast majority of NATO defense spending,” Trump said as he met with Stoltenberg at the White House on Tuesday. “And we really cannot rely on one nation to defend all. If you look at it, the disproportionality of what the United States is doing is really too great, but we’re working on that.”

Stoltenberg is the first NATO chief to address Congress. He drew a crowded but not full house that included Pence, Joint Chiefs of Staff chairman Joseph F. Dunford Jr. and several Cabinet members.

Democrats packed one side of the House chamber, but there were about two dozen empty seats on the Republican side.

Trump was not there, and neither was Secretary of State Mike Pompeo, who has echoed Trump’s nationalist critiques of international organizations but also called the NATO alliance an “indispensable institution.”

Pompeo’s public calendar showed no conflict during the Stoltenberg address. In response to a question about why he did not attend, a State Department spokesperson noted only that Pompeo was meeting with Stoltenberg separately this week.

Stoltenberg insisted that the differences of opinion, like the differences in NATO members’ geography and history, could be an asset.

“Open discussions and different views is not a sign of weakness,” he said. “It is a sign of strength.”

As Trump does, Stoltenberg also blurred some distinctions between the NATO military alliance and the other economic and political bonds between the United States and Europe, noting that they are each other’s largest trading partner.

He did not mention the current U.S. and NATO tension with alliance member Turkey over its planned purchase of Russian missiles. Republicans and Democrats backed the Trump administration’s stance that Turkey may not purchase both those missiles and the U.S.-made F-35 fighter jet.

“I have a lot of concerns with Turkey having those missiles, Sen. John Barrasso (R-Wyo.) said after Stoltenberg’s address. “I’m opposed to them having the Russian ability as part of their defense system.”

Sen. Christopher A. Coons (D-Del.) said, “I think it is a fundamental challenge to NATO’s collective security model.”

Turkish leaders have accused the United States of risking NATO security by objecting to their purchase of the Russian missile defense system.

In a tweet a few hours after Stoltenberg’s speech Wednesday, Turkish Vice President Fuat Oktay wrote: “The United States must choose. Does it want to remain Turkey’s ally or risk our friendship by joining forces with terrorists to undermine its NATO ally’s defense against its enemies.”