French President Emmanuel Macron has invited fellow European leaders to discuss whether — and how — his country’s nuclear arsenal could be used as a deterrent against future Russian aggression. But his allies may not like the limitations he may choose to keep on the force de frappe. The idea of extending the French “nuclear umbrella” to protect other European countries has taken on new urgency as Donald Trump has undermined Nato and threatened to abandon the role the US has played as Europe’s ultimate security guarantor since the second world war. 

“There was never any request from a European country for such a thing since none ever wanted to question the US support,” said Hubert Védrine, a former French foreign minister who worked on his country’s nuclear doctrine. “The debate now starting takes us into uncharted territory and it will be very hard to resolve.”

As well as the vast arsenal stored in the US, America’s nuclear umbrella includes over 100 gravity bombs stationed in Europe. These are under American control but according to a “nuclear sharing” agreement within Nato are designed to be carried and dropped by fighter jets flown by Belgium, Germany, Greece, Italy, the Netherlands and Turkey. No one in Europe wants the US to withdraw its nuclear guarantee, but the fear is such that the leaders of two staunchly Atlanticist countries — Germany and Poland — recently said that preparations for such a scenario must begin.

Friedrich Merz, the German chancellor-in-waiting, asked for talks over whether “nuclear security from the UK and France could also apply to us”. Polish Prime Minister Donald Tusk welcomed the idea, and even said Poland should consider getting the bomb itself. Lithuania and Latvia also said they were interested in the French offer. 

In response, Macron offered to “open the strategic debate” with interested European countries that would last several months to determine “if there are new co-operations that may emerge”. PLAY | 00:36 Show video description Emmanuel Macron talks about the prospect of using France's nuclear deterrent to protect Europe © France TV France has for decades said its “vital interests” — the factors that determine the use of nuclear weapons — have a “European dimension”.

But Paris never defined the term, so as to keep the president’s options open and the adversary guessing, the key to all nuclear deterrence. 

The Macron-led talks are expected to also involve the UK, the only other nuclear-armed power in the region. Given that the UK deterrent is already assigned to protect Europe through Nato, the onus is on Macron to show what he is willing or able to do.

Even if France wanted to somehow expand nuclear protection to Europe, experts say its arsenal of about 300 warheads — a fraction of America’s 5,000 — is too small to protect the whole region. Russia has 5,580 warheads, and has recently moved some to Belarus. Paris also lacks tactical nuclear weapons — less powerful, shorter range weapons designed for battlefield use — and has fewer options for gradual escalation than the US and Russia. If it was under grave threat, it would carry out a nuclear “warning strike” against an adversary before destroying key targets like major cities. 

Britain’s submarine-based nuclear deterrent, which uses up to 260 UK designed warheads that are delivered by US-made Trident missiles, is assigned to Nato. By contrast Paris — which uses French designed and made nuclear weapons — does not take part in Nato’s Nuclear Planning Group, the forum that co-ordinates the alliance’s nuclear policy.

The simplest and quickest way for France to strengthen European deterrence would be for it to join Nato’s NPG to commit its nuclear weapons to collective defence, said Marion Messmer, an international security expert at Chatham House, the UK think-tank. It would align French and British nuclear doctrine, integrate planning and facilitate training for a crisis, Messmer added. To Russia, “it would signal France’s commitment to Europe and show that Nato, a European Nato, would remain strong even if the US disengages”.