DAVID Cameron headed to the House of Commons today to unveil the new shape of Britain's armed forces. For an hour and a half he fielded questions from MPs about planned cuts to the three services, vowing to all comers that Britain would still be able to project power across the world. It was a deft performance, but the truly startling part for me was hearing a Conservative prime minister say, not once but repeatedly, that Britain's future clout lay in working with its two closest allies, "the United States and France." Playing down the fact that from now until 2019 the cuts mean that Britain will not be able to fly fighter jets off an aircraft carrier, Mr Cameron specifically noted that at least one of two new aircraft carriers under construction would be redesigned with catapults so that it could take American and French aircraft.

Asked by an MP what had changed to make Britain so keen to work with France, the prime minister said that (a) President Nicolas Sarkozy was very keen on this planned cooperation, (b) Mr Sarkozy had shown willing by putting France back into the military command structures of NATO and (c) that France and Britain were both determined to maintain and enhance their defence capabilities. To translate these cautious words into plain English, Mr Cameron was telling MPs: (a) France is a serious military power, indeed the only other serious military power in Europe (b) Mr Sarkozy is a radical pragmatist whose decision to rejoin NATO's military structures buried decades of Gaullist anti-Americanism and (c) like Britain, France is broke.

I confess that I have been sceptical about the idea of Franco-British military cooperation for quite some time. I was working in Washington DC in the run-up to the Iraq war in 2003, and watched as American and British politicians and officials fulminated against French-led opposition to the invasion of Iraq. I was in Brussels as Mr Sarkozy's predecessor, Jacques Chirac, spent the last couple of years of his time in office wallowing in anti-Americanism and none too subtle calls for a two speed Europe with "nos amis britanniques" pushed firmly into the outer core. I heard Mr Sarkozy and other French leaders calling for urgent progress on creating a "Europe of defence", with its own military headquarters in Brussels, pooled EU procurement of kit and ambitious common projects, and knew how such calls were anathema to Britain. I watched Mr Sarkozy drop an early scepticism towards Russia, and suggest that Europe should forge a strategic partnership with Moscow, symbolised by his desire to sell advanced amphibious ships to Russia.

Only this summer, I listened as a senior French figure grumbled mightily about how Britain was failing to keep its side of the bargain when it came to allowing much more ambitious EU-flagged defence initiatives. That had been the quid pro quo when France rejoined NATO's military command, he said. By dragging its feet, Britain was putting France in a difficult position.

I think I may have been too sceptical, at least I think so for now. Hugely ambitious ideas are to be heard flying around, as this newspaper's defence and security editor reports this week. Tonight, after Mr Cameron made his Commons statement, the Elysée put out its own press release, saying Mr Sarkozy had learned of the British government's strategic defence and security review, as announced by Mr Cameron, with the "greatest interest":

The decisions announced are courageous and demonstrate the willingness of the United Kingdom fully to carry out her responsibilities when it comes to the security of the international community and our Allies. This willingness is shared by France. The United Kingdom is offering France a particularly close partnership in the field of defence and security. France is greatly in favour of this idea, and will work to make it a reality.

That is a seriously pragmatic piece of French officialese, for one overwhelming reason. The statement does not mention the EU or Europe once. That marks a big break with years of French diplomatic and political rhetoric, and it is a sign of how much Mr Sarkozy wants to make this work.

Now, some may object that France has always had a double-edged relationship with the EU.It is true that France has always combined loud public protestations of European selflessness with a flinty focus on her own national interests. But a big part of the French diplomatic and political machine really is steeped in the idea that future French global clout can only be preserved by working with and through Europe (albeit a Europe that thinks and acts a lot like a big France, with Germany paying the bills). British neuralgia towards Europe was not something to be indulged, but something to be marginalised.

What is going on? The evidence suggests that Mr Cameron and Mr Sarkozy, who are not obvious soulmates, have discovered they have two extremely important things in common. They are strikingly uninterested in ideology. And when the conditions are right, they are risk takers.

There are parallels here, I would argue, with Mr Cameron's decision to offer Nick Clegg and the Liberal Democrats a full coalition government. That too was a piece of pragmatism married to a taste for big gambles. It has worked out well so far, confounding the sceptics. But there is also one big difference between the coalition and Franco-British defence cooperation. The coalition has only to last until 2015 for it to be declared a success: that is the time frame agreed for cooperation between Mr Clegg and Mr Cameron, and it is reasonable to assume that 2015 will only be reached if Mr Cameron and Mr Clegg remain leaders of their respective parties. The coalition rests very fimly on those two men, who share not just a rather centrist set of political beliefs, but a smooth, pragmatic, un-ideological approach to politics in general.

Franco-British defence cooperation is equally bound up with the happy accident of where Mr Sarkozy and Mr Cameron find themselves just now: two bold and pragmatic men struggling to maintain global hard power with no money in the bank. But aircraft carriers and the like have long lifetimes: much longer than any democratic European politician can hope to stay in office. I struggle, personally, to imagine how this cooperation could ever have taken off with Mr Chirac still in charge of France. I struggle to see how it could survive if Mr Sarkozy loses the next election in 2012 to a Socialist: Mr Cameron working with President Aubry, anyone? So just how sustainable is this cooperation over a 20 or 30 year period? I don't know, is the honest answer. My initial blanket scepticism now looks like it was exaggerated. But can this last? I am not sure.