It was meant to be a celebration of the 70th anniversary of NATO. But the mood at a gathering of the alliance’s leaders this week in London was anything but celebratory. There was no disguising the fact that NATO is in deep — even existential — trouble.

Nothing illustrated this unease more than the brief elation among NATO hands over U.S. President Donald Trump’s surprising defense of the alliance. If Trump, previously a NATO skeptic, has had a change of heart, perhaps things are not so dire after all.

Except Trump didn’t have a change of heart. His defense of the alliance was little more than a rhetorical stick with which to beat French President Emmanuel Macron, who had previously bemoaned the institution’s “brain death.”

Credit must also go to NATO Secretary-General Jens Stoltenberg, who has done a masterful job of managing Trump, calming the president’s tantrums and stroking his ego. Witness Stoltenberg’s eagerness, at a joint news conference, to credit Trump for increased military spending by many members. “Your leadership on defense spending is having a real impact,” he purred, as the president preened.

But Trump’s skepticism was soon on display once again. He carped about Germany’s paltry defense spending, threatened trade penalties against members who don’t pony up and refused to commit to Article 5 of the North Atlantic Treaty, the all-for-one clause that requires the group to defend any member under attack.

Trump is Trump: There’s every possibility that he’ll arrive at the next NATO summit brandishing a flamethrower instead of the pea shooter he wielded in London.

And not even Stoltenberg’s diplomatic skills can mask the fundamental problems — both philosophical and practical — that bedevil the alliance. These go beyond the previous spasms of disunity NATO has endured, such as the 1966 French withdrawal from the integrated military command or Turkey’s 1974 invasion of Cyprus and showdown with Greece.

An important reason NATO has held together for 70 years is the common perception of the threat posed by the Soviet Union and, more recently, Russia. But core members of the alliance no longer agree on this. Turkey is buying arms from Moscow, including missile systems that endanger NATO defenses; Turkish President Recep Tayyip Erdogan is more suspicious about America’s intentions than Russia’s. Trump has been cozying up to Russian President Vladimir Putin at every opportunity, against the advice of his entire military and intelligence community. Macron is ambivalent at best.

Nor is there consensus over the other threats to the West: terrorism, cyberwarfare and China. Turkey and France have accused each other of harboring terrorists. The divisions over Russia compromise the alliance’s ability to defend against online attacks. And some NATO members see China more as a source of money than menace.

Even if most member states were to agree on what constitutes a threat, it doesn’t automatically follow that NATO is the best shield. Cyberwarfare requires subtler, more secretive approaches than the alliance is capable of deploying, and counterterrorism calls for swift, supple responses — not the kind of thing you leave to a giant multinational bureaucracy. Taking on China may require a North Pacific Treaty Organization, in which European militaries would play only a small part.

This doesn’t mean NATO is no longer relevant. It is relevant for the same reason it was at its founding: the potent threat of Moscow. Getting all the members to recognize this, and to act accordingly, will require much more than Stoltenberg’s diplomatic guile — it will take political leadership of a high order from the most powerful members. Nothing said or done in London this week suggested such leadership is at hand.

Bloomberg View columnist Bobby Ghosh writes on foreign affairs.

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