In France, for centuries, the idea of sharing military resources with perfidious Albion would have been unthinkable.
But the two countries have buried ancient enmities, and the prevailing view in France is that the relationship should nowadays be pragmatic and based firmly on realpolitik.
"We are on the verge of becoming a second-rank power on the world stage," laments French defence analyst Bruno Carre.
"The only way we can remain a medium-first-rank power is by increasing relations with our best allies."
Dr Carre says the term "best allies" well describes Britain and France - two well matched powers, with similar priorities, who are "the only two nations sending troops all the time, everywhere".
Of course, the relationship between the French and the British has always been a curious mix of mutual suspicion and admiration.
And the defence relationship has also been double-edged. The 1904 Entente Cordiale brought an end to centuries of regular wars, and the two countries have been allies ever since, most notably in the two world wars.
But there have been rocky patches since then.
The Suez Crisis of 1956 could have been a joint triumph, but their military success in seizing the Canal in Egypt turned to humiliation as they were forced to withdraw under international pressure.
In 2003, France split dramatically from Britain and the United States, leading attempts to prevent their invasion of Iraq.
But despite these convulsions, there have been times of close co-operation - in the first Gulf War, in Bosnia and now Afghanistan.
Dr Carre does not believe that Britain and France would sign any deal that would compromise their ability to act independently if they found themselves on opposite sides of an argument, as in 2003.
Nor does he think that the two militaries will be vying with each other to do better out of the deal. He says the two sides are increasingly close.
"What is really being done is we are building on trust - the main problem between Britain and France for 1,000 years has been mistrust. Sarkozy and Cameron are trying to get rid of that.
"It must be a good deal for both countries - if it's not, it will fail."
However, there are some qualms in France, not least about co-operating on nuclear technology.
"Some people point out that France's nuclear deterrent is more independent than the British one, which couldn't exist without American knowledge," says Jean Jaffre, a French commentator based in London.
It is also being opposed by some on the left, who have long desired more pan-European military integration.
They see this pact as France buying into Britain's Atlanticist strategy, rather than Britain adopting a continental approach, says Mr Jaffre.
That concern extends beyond France, he adds, with some fearing a setback for European integration.
"The fear of many on the continent is that by co-operating with Britain, this will prevent any EU military strategy and push defence towards the American perspective," he says.
Britain and France have been trying to reinforce defence co-operation for years. In 1998, British Prime Minister Tony Blair and French President Jacques Chirac declared a new era in military relations - before that was derailed by their face-off over Iraq.
But the imperatives have not really changed. The two countries want both to project power and save money - and have decided that means burying the hatchet and working together.