President Trump has rekindled the “madman” approach to foreign policy, projecting unpredictability, like when he speculated about destroying North Korea as part of a long-running effort to forge peace talks. He has also taken a tough stance against Iran, musing about shooting its boats “out of the water” while pulling out of a nuclear agreement with that country.

But while the madman theory isn't entirely new, it's generally reserved for one's enemies. And Trump, increasingly, is employing a version of it against our allies, as well.

Perhaps the starkest example came early Wednesday at a NATO summit in Brussels, when Trump accused Germany of being a “captive” of Russia because of its increasing dependence on Russian energy. The pointed exchange with NATO Secretary General Jens Stoltenberg seemed to catch those in the room — including the Americans — by surprise for its provocativeness.

Here's an abridged version (with highlights bolded):

TRUMP: Well, I have to say, I think it's very sad when Germany makes a massive oil and gas deal with Russia, where you're supposed to be guarding against Russia, and Germany goes out and pays billions and billions of dollars a year to Russia. So we're protecting Germany. We're protecting France. We're protecting all of these countries. And then numerous of the countries go out and make a pipeline deal with Russia, where they're paying billions of dollars into the coffers of Russia.
So we're supposed to protect you against Russia, but they're paying billions of dollars to Russia, and I think that's very inappropriate.
. . .
STOLTENBERG: You know, NATO is an alliance of 29 nations, and there are sometimes differences and different views, and also some disagreements. And the gas pipeline from Russia to Germany is one issue where allies disagree. But the strength of NATO is that despite these differences, we have always been able to unite around our core task, to protect and defend each other, because we understand that we are stronger together than apart. I think that two World Wars and the Cold War taught us that we are stronger together than apart.
TRUMP: But how can you be together when a country is getting its energy from the person you want protection against or from the group that you want protection?
STOLTENBERG: Because we understand that when we stand together, also in dealing with Russia, we are stronger. I think what we have seen is that —
PRESIDENT TRUMP: No, you're just making Russia richer. You're not dealing with Russia. You're making Russia richer. . . . But Germany, as far as I'm concerned, is captive to Russia, because it's getting so much of its energy from Russia. So we're supposed to protect Germany, but they're getting their energy from Russia. Explain that. And it can't be explained; you know that.

It's easy to dismiss this as Trump spouting off at an inopportune and potentially embarrassing time — even being undiplomatic. But it's pretty clear what the play is here. Trump has long expressed concerns about how much European countries are contributing to the common defense that is NATO. This is an American concern that predates his presidency, but he seems hellbent on bringing it to the forefront, even if it requires spoiling the party.

And the subject of his angst, Germany, is a convenient one, for a few reasons. First, Trump has long decried its open-borders approach to refugees as the antithesis of the approach the Western world needs. Second, its chancellor, Angela Merkel, is struggling to hold together her governing coalition. And last, this was already an issue that pitted NATO countries — especially those in Eastern Europe — against Germany. It was sitting there waiting for Trump to slam a wedge right down the middle of NATO, and he did.

(For a great explainer on the Germany-Russia pipeline situation, check out this from The Post's Worldviews team.)

Plenty of Trump's opponents have noted that this is exactly the kind of thing Russian President Vladimir Putin would want festering inside the coalition that was forged largely to protect against his country. And that's certainly true. But it's also clear that Trump is trying to get something out of this: increased NATO funding from these countries, which Trump is now reportedly talking about doubling from 2 percent of their respective GDPs to 4 percent. And if they don't toe his line, he's apparently willing to break some stuff — verbally, at least.

It may not be the kind of genteel diplomacy that lots of people want or expect, but that's what it is. And as with every use of the “madman” approach, the bigger rewards go hand in hand with the bigger risks.