Christina Mackenzie Paris and Angus Batey London

It was around 1990 when a retired, very senior officer was giving a history lecture to a class of British officer candidates, reminding them that the 20th was the first century since the Norman Conquest of 1066 that Britain and France had never been at war with one another.

“There's still 10 years left,” came a voice from the back rows.

Britain and France are, in defense terms, the most similar countries in Europe. They spend about the same percentage of their GDP on defense and together account for almost half of military spending in Europe. Their armed forces are of similar strength (228,656 for France; 173,020 for the U.K.). They are Europe's two nuclear powers and were the main actors in the 2011 Libya campaign. Culturally, more Britons speak French than any other foreign language. Two major players in both defense industries—MBDA and Thales—are already cross-channel companies.

But the defense relationship has been fractious. The U.K has had close ties with the U.S., particularly on nuclear and intelligence matters, but France was long absent from NATO. When it came to major combat aircraft programs, Britain and Germany collaborated and France went its own way. Later, BAE Systems' transatlantic and global strategy ran counter to the Franco-German formation of EADS.

In 2012, however, it was Germany's turn to torpedo a Eurocentric industrial strategy: the EADS-BAE Systems merger. The idea made perfect sense. BAE Systems is too reliant on military sales and EADS on civilian business, notably Airbus. Both were seeking to redress the balance. But Germany, where the majority of EADS's military division, Cassidian, is based, felt threatened. Even if EADS's CEO Tom Enders is German, the answer from his government was a firm “nein”—delivered, rather embarrassingly, after the companies had declared their intention to merge.

But if “static” is the best way to describe defense cooperation in Europe's industrial sphere, that is not the case among the armed forces of France and Britain where sometimes personal initiatives, and sometimes government steps, such as the signature in November 2010 of the Lancaster House treaties, have led to a positive trend in cooperation.

One of the objectives in those treaties is to set up a Combined Joint Expeditionary Force (CJEF), composed of two contingents of 5,000 from the army, navy and air forces of each country, plus a carrier strike group. The CJEF would be at the disposal of France and the U.K. but would also be available for NATO or European Union missions.