Not long after President Donald Trump woke up Wednesday morning in Brussels, he arrived at a public breakfast and took aim at his latest favorite target for ridicule, NATO allies, for not boosting their defense spending.

“Many countries are not paying what they should,” he said. “And, frankly, many countries owe us a tremendous amount of money for many years back, where they’re delinquent, as far as I’m concerned, because the United States has had to pay for them. So if you go back 10 or 20 years, you’ll just add it all up. It’s massive amounts of money is owed.”

The president’s message was obviously intended jolt the other 28 NATO members and put them on-notice that the White House will not back down from demands to pay more for the military alliance.

But it was jarring for another reason: NATO doesn’t work this way.

The North Atlantic Treaty Organization, which has been a keystone of global security since its formation in 1949, does not involve a membership that collects dues. There is no annual subscription.

“President Trump does not appear to understand that the 2% of GDP spending by the allies is a guideline, not a mandate,” said James Stavridis, retired U.S. Navy admiral who commanded all NATO forces and is now dean of the Fletcher School of Law and Diplomacy at Tufts University. “He tends to liken the situation to a need to hound golfers for not paying their dues at the local country club. While it makes sense to pressure the Europeans to hit the 2% goal, we must avoid splitting the alliance over the issue.”

Daniel Fried, a former Assistant Secretary of State for European and Eurasian Affairs and Ambassador to Poland, said Trump appears intent on rattling NATO members. Before Trump embarked for Brussels, he criticized the allies on Twitter at dawn, and then reemphasized the point again about an hour later. He delivered a similar message in front of television cameras on the White House lawn, before pecking out another rough tweet while aboard Air Force One as it soared eastward above the Atlantic Ocean.

“The question is whether the president is doing this to make NATO better, or if he doing it because he doesn’t like NATO and wants to see its demise?” Fried said. “The latter is a depressing question to ponder when it involves a sitting commander-in-chief.”

NATO’s defining Article 5 principle for collective defense states that an attack on one nation is an attack on them all and was invoked only once, by North American and European allies after the Sept. 11 attacks on the U.S. Article 5’s mutual defense is why it’s important that NATO nations spend money on weapons and soldiers to ensure they’re prepared to defend themselves – particularly in the face of Russian aggression.

Although Trump treats the 2% spending mark as if it were an international mandate, it is not.

The figure was established in 2002 when members agreed upon a non-binding target to contribute 2% of Gross Domestic Product to collectively share the burden of defense costs. It was further ensconced in 2014 at a summit in Wales when all the nations that were not meeting the 2% target made a pledged to reach the mark within a decade.

Jorge Benitez, NATO expert at the Atlantic Council, a Washington-based think tank, said Trump has made it clear that he misunderstands two key facts about NATO.

“Trump does not understand that each ally spending 2% of GDP on defense is a goal, not a debt,” he said. “It is a pledge for common action, not a loan from the United States. Trump also does not understand that the 2% target is for national defense spending, not money owed to the U.S.”

The president has repeated his inaccurate view that NATO allies owe money and that they owe it to the U.S. whenever he discusses the topic. However inaccurately worded, Trump’s tough talk may have resulted in the desired effect: Inspiring allies to pay more.

Jens Stoltenberg, Secretary General of NATO since 2014, has said the allies have achieved a record increase in defense spending. By the end of the year, he expects eight members — the U.S., Britain, Estonia, Greece, Lithuania, Latvia, Poland, and Romania — will reach the 2% mark. That’s compared to just three allies in 2014.

Still, the U.S. accounts for 22% of the NATO alliance’s common funding, which is spent on projects like military readiness, joint exercises, and initiatives to counter cyber-warfare. And there has long been widespread recognition among both Republicans and Democrats that NATO members should step up their spending. The criticism can be traced as far back as 1953, when Secretary of State John Foster Dulles notoriously threatened that the U.S. would embark on “an agonizing reappraisal” of U.S. military support if European nations didn’t show a willingness to defend themselves against the then-Soviet Union.

The driving need for robust defense spending waned a bit in 1989 with the fall of the Berlin Wall and the end of the Cold War. But returned in 2014 after Russia seized Crimea and backed armed separatists in eastern Ukraine, causing President Barack Obama to also pressure NATO allies to increase their defense spending.

U.S. NATO Ambassador Kay Bailey Hutchison told “Fox News Sunday” that she believes Russia is “trying to flip many of our allies. They want to destabilize the strongest defense alliance in the history of the world, and that’s NATO.”

Meanwhile, the Trump administration continues to pour money and manpower into Europe with the aim of strengthening allies against Russian aggression and re-building the American footprint that withered after the Cold War ended.

The Pentagon’s European Deterrence Initiative, or EDI, will spend $4.8 billion this fiscal year — and is set to grow to $6.5 billion in 2019. The Pentagon is refurbishing facilities, airfields and training ranges in across eastern Europe Last year, the Army sent 87 tanks, 144 armored vehicles and 3,500 troops to Poland in the biggest U.S. military deployment on the continent in decades.

European Council President Donald Tusk said Tuesday that it was critical that Europeans continue to increase spending. But as a Polish national who grew up behind the Iron Curtain, he attempted to relay the importance of allies to Trump by stressing that the U.S. “does not have, and will not have, a better ally than Europe.”

“Dear America,” he said, “appreciate your allies, after all you don’t have all that many.”

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Write to W.J. Hennigan at william.hennigan@time.com.