‘Three presidents, three visions as Nato gathers for a shot in the arm — or the head’, by Michael Clarke, published in The Sunday Times.

London is preparing to host this week’s Nato meeting with all its usual down-to-earth sang-froid. An evening reception at Buckingham Palace, then a morning of side meetings and cabals, followed by a three-hour session of all 29 leaders, before they rush back to their own troubled countries and a declaration is unveiled.

What could possibly go wrong? Quite a lot actually.

British officials have prepared in the hope that this meeting will pass off smoothly and give the transatlantic alliance a shot in the arm. It may nevertheless be known as the “summit of the three presidents”, and looks more likely to give Nato a shot in the head. Donald Trump, Emmanuel Macron and Recep Tayyip Erdogan will bring with them three very different visions of what Nato should be in the 2020s. It will take more than Whitehall choreography to paper it over.

The whole event has been pared down to make it less likely that the US president will wreck it — just a 36-hour bash, not even a full summit. It will be a “leaders’ meeting”, and with plenty of royal attention, if only to keep Trump sweet.

In truth, even Trump is not Nato’s main problem these days. He is certainly no fan of the alliance, and because he sees managing money as synonymous with managing politics, he never will be. But he speaks from a narrowing apex of the US political establishment, and opinion on either side of the Atlantic is not as out of kilter as it sometimes seems. The US could easily renew its “grand bargain” with Europe — or even redesign it completely for the 21st century — if only the Europeans knew what they wanted and would genuinely commit to it. Therein lies the problem that the other two presidents will bring with them.

Europe has been “structurally disarming” since the end of the Cold War. If Britain leaves the EU next year, that will bring to 80% the proportion of Nato’s defence expenditure being met by the non-EU countries in the alliance. You can see why Washington keeps making the point as trade protectionism grows.

But more than that, Nato has struggled to maintain the basic political consensus that backs up any use of military force, despite — or perhaps because — it has doubled its membership since 1995 to embrace the new democracies of Europe.

The Libya crisis in 2011 should have been a warning that even successful military operations can leave Nato politically weaker. It was a real hokey cokey. The US was in, then it was out.

The Germans were out, then they were partly in. Turkey was out, then it was in. The Italians were in, then looked to get out. The only truly consistent player in the war was Colonel Muammar Gadaffi, and he was insane.

The direct effects of Libya and then disturbances across the Sahel have been putting refugee pressures on the southern flank of both Nato and the EU, which exacerbates the gulf between the security agendas of the southern European states and those in the north.

In the north, the fear is revisionist Russia with its military modernisation programmes that make Nato’s conventional forces look inadequate, even though they are more numerous.

In the south, the fear is of instability in the Mediterranean and the Middle East, of uncontrollable numbers of refugees, of economic collapse in another global crisis and the blandishments of Chinese investment that disguise its manipulative nature.

So Erdogan comes to the meeting with a frankly eccentric southern security agenda that bears little relation to anything Nato stands for — not least the protection of democratic principles as envisaged 70 years ago at the founding of the organisation. Turkey now operates in loose alliance with Russia and Iran to uphold the Assad regime in Syria and kick western forces out of the reckoning.

Turkey has invaded northern Syria, both before and after Trump’s winks that it could. It has turned on the Kurdish allies that Nato forces were training and backing. And it’s in the process of deploying Russia’s S-400 air defence system, which will compromise Nato air operations over the eastern Mediterranean.

Diplomats recommend playing it long on Turkish behaviour. Defence analysts wonder what more a country has to do to be asked to leave a military alliance.

And then, we’ll always have Paris. Macron arrives in London not just staunchly defending his view that Nato is “strategically brain dead”, but arguing that a new strategy should be based on a different conception of what’s at stake in European security.

Russia and China are not natural adversaries, he says, when the common enemy is terrorism. This raises predicable hackles in Germany and across the east European countries with their naturally northern agendas.

The argument leaves analysts scratching their heads too. Terrorism is a threat, but is not strategically significant. Whereas everything Russia and China have done in the past 10 years elevates their behaviour to that of a “strategic adversary”. It would be foolish to underestimate this.

However, with Germany so reluctant to play a central role in the military defence of Europe, Macron is moving to occupy the strategic space he perceives that Brexit Britain and Germany (after Angela Merkel) are leaving him. He would redesign European security around a new EU-Nato relationship and promote a different type of transatlantic link. But there are simply too many moving parts here, even if he knew what the final result would look like. And though many are inclined to agree with his diagnosis, few support his opaque cures.

Which leaves three presidents talking past each other this week, 26 other national leaders laying out their stall, and British officials working hard. Britain has arranged for France, Germany and Turkey to join it in a private meeting before the grand session on Wednesday.

If a summit inside a summit can make a difference, this is where we might eventually see it.

It’s a sign of the times that Trump will be doing something else.

Professor Michael Clarke was director-general of the Royal United Services Institute from 2007-15. His book, Tipping Point: Britain, Brexit and Security in the 2020s, was published last week (IB Tauris)