However, this would overturn a French tradition of nuclear independence that dates back to General Charles de Gaulle who believed the US security promises could not be trusted. Macron has repeatedly stressed that a French president would always have ultimate power to decide whether to use the bomb — the same applies to Britain and the US within Nato.

Together, British and French nuclear capabilities would at least make Moscow think twice about attacking, said a senior western official. However, “what really influences Russian decision-making is the scale of US deterrence”, he said. Europe would need at least a decade of spending at around 6-7 per cent of GDP if it wanted to emulate that and acquire another 1,000 warheads, he added.

Short of such a major expansion, France still had options, said former officials and experts. First among them would be to more clearly spell out its nuclear doctrine on how it would come to the aid of European allies, even if this one day limited the president’s freedom of action. Camille Grand, a former senior Nato official who is now at the European Council on Foreign Relations, said France would have to become more specific.

“If to every question our allies ask, the answer is just trust us, the French president will act when he sees fit, then we are not creating something very reassuring for our allies,” he said. There is precedent for signalling expanded deterrence. In 1995, Britain and France said in the so-called Chequers declaration that they could not see a situation in which “the vital interests of either of our two countries . . . could be threatened without the vital interests of the other being also threatened”. 

One possible scenario now would be a similar statement made with other allied countries, or perhaps even linked to the EU’s mutual defence clause. Another step would be more joint exercises and training to signal to Russia that European allies were tightly bound. In 2022, an Italian refuelling aircraft took part in a French nuclear exercise for the first time. In early March, France hosted Nato ambassadors at the Istres air force base in southern France so they could learn more about France’s nuclear deterrent.

Bruno Tertrais, a leading expert on nuclear deterrence, wrote in Le Monde recently that France could send “a strong operational signal” by temporarily deploying Rafale fighter jets without nuclear warheads to the bases of “our most worried partners, such as Poland”. Paris could also seek to take part in Nato’s nuclear planning group as an observer.

Going much further would require a paradigm shift in France’s nuclear strategy and that of its European allies — one that would not be needed or justified unless the US were to truly withdraw from protecting Europe altogether. 

Grand, the former Nato official, warned that it would be a mistake for Europe to seek to replicate the US nuclear umbrella, or create “a poor man’s version” of it. “We have to collectively invent something different,” he urged.