In French military circles they are known as the "crown jewels," the fleet of Mirage fighter jets kept in the sky and devoted to delivering a nuclear strike. In Britain, the four Scottish-based submarines armed with 200 Trident nuclear warheads are considered untouchable.
So when the leaders of France and Britain agreed to combine military operations - including an astonishing deal to unite testing and maintenance of their nuclear arsenals - many senior military figures in both countries were aghast. There was talk, on both sides of the Channel, of Waterloo, Trafalgar, Agincourt and more recent instances of Anglo-French discord.
The agreement was nonetheless signed on Tuesday by President Nicolas Sarkozy and Prime Minister David Cameron, both of whom are struggling with deep budget cuts and sagging economies. It will place British and French special forces together in a joint force of 10,000 troops and permit the sharing of aircraft carriers, unmanned drone aircraft and other military hardware. Most controversially, it will combine the testing and oversight of the two countries' nuclear arsenals at a joint facility near Dijon, France.
Mr. Sarkozy, who has long called for a united European defence force, described the treaty in euphoric terms: "This is a decision that is unprecedented," he said, "and it shows a level of co-operation and confidence between our two nations that is unique in history."
In Britain, where Mr. Cameron described the deal strictly in cost-saving terms ("The treaty is based on pragmatism, not just sentiment," he said), the prospect of Anglo-French co-operation was met with open contempt from senior officers, Tory backbenchers and the right-wing media. London's Daily Express ran a banner front-page headline, British Army Under French Orders.
That was a reference to the prospect of British troops serving under the command of a French general, an event likely to occur at some point under the treaty.
Although it has been in the works for months and was initiated by former prime minister Gordon Brown's Labour government, the agreement follows a drastic defence-spending cut by Mr. Cameron's Conservative-Liberal coalition that will see Britain eliminate its only jet fighter capable of landing on an aircraft carrier while building two carriers - one of which will be immediately mothballed - that will not come into service until 2020. The agreement represents an effort by both countries to remain global military players in the face of such cuts.
A few years ago, such a deal would have been unthinkable. But France has rejoined the senior command of NATO after a four-decade absence, and its refusal to participate in the Iraq war no longer outrages Downing Street. The prospect of an entente cordiale, or even an entente budgétaire, is now politically acceptable, at least to Mr. Cameron's circle of moderates.
In some military and political circles, though, the treaty has drawn howls of alarm.
Colonel Tim Collins, a well-known figure retired from the Iraq war, denounced the decision, describing the French as an untrustworthy partner that armed the Argentines against the British in the Falklands War, betrayed its NATO colleagues to the Serbs in Bosnia and couldn't be trusted on missions hunting war-crimes suspects.
"Well it seems now we are to be one with them - at least militarily - and I must admit I am skeptical," Col. Collins said. "The truth is that for years, the French have punched below their weight."
Retired Commander John Muxworthy, head of Britain's military lobby the National Defence Association, scorned the pact: "That is no way to run a country's defence. In World War II we were supposed to be standing side by side with the French and then look what happened."
Veteran Tory backbencher Bernard Jenkin, a former defence critic and key figure in the party's right-wing faction, was one of several MPs who openly defied Mr. Cameron. "We cannot have a strategic fusion with a country that has historically, and still has, diametrically different strategic objectives on the world stage," he told Parliament.
France's politicians were more sanguine about the treaty, which has been a goal of some senior commanders in both countries for years. The prospect of nuclear-defence co-operation, however, has been hotly condemned by senior French defence officials, who see the force de frappe as the foundation of their country independence in military matters and a capability that allows France to play the role of a first-rank world power.
To assuage such officials, Mr. Sarkozy's office put out a memo noting that the nuclear co-operation will be strictly technical, not operational, and will accord "full respect for the independence of the nuclear forces of both countries."