When it comes to international diplomacy, President Donald Trump is no milquetoast mumbler. He can be undiplomatically blunt, even rude, but that doesn’t automatically mean he’s wrong. Take, for instance, the longstanding dispute between the United States and its NATO allies over how much everyone contributes to the collective defense of Europe.

At the NATO summit in Brussels on Wednesday, Trump scorched the sensibilities of NATO Secretary General Jens Stoltenberg and German Chancellor Angela Merkel with this zinger: “Germany, as far as I’m concerned, is captive to Russia because it’s getting so much of its energy from Russia.”

The Washington Post described Trump’s remarks as “among the harshest in the history of the post-World War II alliance.” Pass the smelling salts.

“We have to talk about the billions and billions of dollars that’s being paid to the country we’re supposed to be protecting you against,” Trump said, referring to European purchases of Russian natural gas.

Why would he say that? Because it’s true, and to Trump (and many other Americans) it’s galling. Germany, one of the wealthiest countries on Earth, is among more than 20 NATO laggards not yet chipping in 2 percent of gross domestic product for defense — as these nations pledged to achieve by 2024.

For the past several years, only a handful of NATO members have cleared the 2 percent bar; the U.S. spends about twice that. This year, however, NATO expects eight of its 29 members to reach 2 percent.

Great, that only leaves 21 more. Is Trump impatient? Yep. And so was President Barack Obama, and President George W. Bush. We’ve often demanded that European countries pony up.

NATO is built on the premise that an attack on one member is an attack on every member. That represents an important, and costly, bulwark against security threats, including terrorism and Russian aggression. The Europeans value American security commitments, but apparently not enough to swiftly fork over more cash.

Thus, Trump’s mission is to shame France, Germany and other European countries into finally contributing their due. “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,” Trump said.

Trump calling important allies “delinquent”? The president does have a way with words. Technically he’s incorrect, since there’s no money owed to the United States. But the larger point is that the defense of Europe is a shared burden. And candor has its place.

Some of NATO’s nervous Nellies fear Trump will rupture the alliance by dissing America’s friends and yielding too much to Russian President Vladimir Putin in a summit scheduled for next week. Yet after Trump rattled the contributions cup in Brussels, he joined NATO leaders in approving sweeping plans to boost the alliance’s defenses against Russia and terrorism.

He also took a final shot at goading NATO deadbeats. He called for NATO countries to boost military spending to 4 percent of GDP.

Europe, over to you. Again.