President Donald Trump told NATO allies on Thursday morning they must contribute their 2 percent GDP commitment to the defense pact, an agreed-upon level that 23 of 28 NATO members do not meet, and may need to start contributing even more.

Trump told a solemn gathering at the new NATO headquarters in Brussels, Belgium, that the alliance needed to make up ground after years of “chronic underpayments.”

“The NATO of the future must include a great focus on terrorism and immigration, as well as threats from Russia, and on NATO’s eastern and southern borders.”

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The issue is made more pressing, Trump said, by recent terrorist attacks, citing the suicide bombing Monday night in Manchester, England. Twenty-two people died in the attack, for which the Islamic State claimed credit.

NATO’s problem is compounded by threats from illegal immigration and new Russian aggression, Trump told NATO leaders.

The scolding of NATO leaders was expected, but Trump was especially assertive in his brief remarks before the European leaders.

Trump stood between two slabs of the fallen and infamous Berlin Wall, which Germany donated to the headquarters, as well as twisted metal beams from the fallen North Tower of the World Trade Center, which crashed to earth on Sept. 11, 2001.

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There he responded to earlier remarks from German Chancellor Angela Merkel — a subtle dig at Trump and his immigration policies — that walls should be torn down.

“The NATO of the future must include a great focus on terrorism and immigration, as well as threats from Russia, and on NATO’s eastern and southern borders,” said Trump. “NATO members must finally contribute their fair share and meet their financial obligations. But 23 of the 28 member nations are still not paying what they should be paying, and what they are supposed to be paying for their defense. This is not fair to the people and the taxpayers of the United States.”

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Trump said the member nations not contributing to their own defense causes huge deficits in common defense readiness.

“Over the last eight years, the United States spent more on defense than all other NATO nations combined, ” Trump said. “If all NATO members had spent just two percent of their (gross domestic product) on defense last year, we would have had another $119 billion for our collective defense and for the financing of additional NATO reserves.”

Trump said the chronic underpayments would be the “bare minimum” to catch up to the problem.

“We should recognize that with these chronic underpayments and growing threats, even 2 percent of GDP is insufficient to close the gaps in modernizing, readiness, and the size of forces,” said Trump. “We have to make up for the many years lost. Two percent is the bare minimum for confronting today’s very real and very vicious threats. If NATO countries made their full and complete contributions, then NATO would be even stronger than it is today, especially from the threat of terrorism.”

The NATO leaders stood stone-faced as Trump rapped their knuckles. Some cracked nervous smiles.

NATO, based in Brussels, is a common-defense treaty organization founded in 1949. It included the United States and Canada as way to counter Soviet and communist aggression after World War II.

The might of the alliance is formidable. With France, the United Kingdom, Italy and Germany, NATO accounts for the vast majority of military might in the world. And it is growing in Eastern Europe. Montenegro will join NATO in June.

But the United States contributes the most to NATO: In 2016, it spent 3.6 percent of its national income on defense. A lot of that money is spent on U.S. bases and training in Germany and the United Kingdom.

The United Kingdom spends about 2 percent on defense. But Germany has slacked: The powerful nation, a key buffer between Russia and the rest of Western Europe, has struggled to spend above 1 percent of its GDP on defense. Trump is reportedly irked at Merkel for not committing to spending more.

Because of the weak European contribution to NATO, it frequently drew Trump’s ire on the 2016 campaign trail.

But last month, Trump held a joint press conference with NATO Secretary-General Jens Stoltenberg, at which he said he longer considers NATO obsolete, because the treaty organization adjusted its anti-terrorism policies.

It was welcome news for European leaders, who are nervous about Russian aggression in the Ukraine. But now, Trump says, they have to pay up, and start rebuilding their defenses.