When François Hollande hands over the launch codes for the force de frappe — France’s independent nuclear deterrent — to Emmanuel Macron on Sunday, he also bequeaths a daunting list of problems. How the new president deals with them will determine not only his country’s security in the coming years, but Europe’s and ours.

Brexit will leave the European Union with only one military heavyweight: France. Its armed forces have advanced capabilities on land, sea and air (and in cyberspace), and a global reach. No other EU country comes close. In many parts of the world — the Pacific and north Africa, for example — France matters more than Britain does.

Like ours, France’s armed forces are thinly stretched. As the Friends of Europe think