France has a nuclear umbrella. Could its European allies fit under it?

So in the end Charles de Gaulle was right.

As president of France in the 1960s, it was he who launched the policy of French strategic independence.

Of course, he said, Americans were more our friends than Russians are. But the US too had interests. And one day their interests would clash with ours.

In the world of today, his warnings have never seemed more clairvoyant.

From his principle of superpower detachment, De Gaulle conjured the notion of France's sovereign nuclear deterrent – whose existence is now at the centre of debates over European security.

France and the UK are the only two countries on the European continent who have nuclear weapons. Currently France has just short of 300 nuclear warheads, which can be fired from France-based aircraft or from submarines.

The UK has about 250. The big difference is that the French arsenal is sovereign – i.e. developed entirely by France – whereas the UK relies on US technical input.

On Wednesday President Emmanuel Macron aired the idea that France's deterrence force (force de frappe) could – in this highly uncertain new era - be associated with the defence of other European countries.

His suggestion drew outrage from politicians of the hard right and left, who say that France is considering "sharing" its nuclear arsenal.

That – according to government officials as well as defence experts – is a falsification of the argument. Nothing is to be "shared".

According to Defence Minister Sébastien Lecornu, the nuclear deterrent "is French and will remain French – from its conception to its production to its operation, under a decision of the president."

What is under discussion is not more fingers on the nuclear button. It is whether France's nuclear protection can be explicitly extended to include other European countries.

Until now French nuclear doctrine has been built around the threat of a massive nuclear response if the president thought the "vital interests" of France were at stake.

The limits of these "vital interests" have always been left deliberately vague – ambiguity and credibility being the two watchwords of nuclear deterrence.

In fact French presidents going back to De Gaulle himself have all hinted that some European countries might de facto already be under the umbrella. In 1964 De Gaulle that France would consider itself threatened if, for example, the USSR attacked Germany.

So in one way there is nothing new in Macron suggesting a European dimension to France's deterrent.

What is new, according to defence analysts, is that for the first time other European countries are also asking for it.

"In the past when France has made overtures [about extending nuclear protection], other countries were reluctant to respond," says Pierre Haroche of the Catholic University of Lille.

"They didn't want to send out the signal that they did not have complete faith in the US and Nato."

"But Trump has clarified the debate," Mr Laroche says. "It's not that the Americans are talking of removing their nuclear deterrent – let's be clear, that does not seem to be on the table right now."

"But the credibility of US nuclear dissuasion is not what it was. That has opened the debate, and led the Germans to look more favourably on the idea of coming under a French and/or British umbrella."

Last month the likely next German chancellor Friedrich Merz surprised the country's partners by saying it might be the moment for discussion with Paris and London on the subject.

How a French or Franco-British European nuclear deterrent might operate is still far from clear.

According to Mr Haroche, one option might be to position French nuclear-armed planes in other countries, such as Germany or Poland. The decision to press the trigger would still rest entirely with the French president, but their presence would send a strong signal.

Alternatively, French bombers could patrol European borders, in the same way they regularly do French borders today. Or airfields could be developed in other countries to which French bombers could quickly deploy in an emergency.

Numbers are an issue. Are 300 French warheads enough against Russia's thousands? Maybe not – but in an alliance with the UK 300 become 550. Also (to repeat the point) the American nuclear deterrent is still in theory in place. There are US nuclear bombs in Germany, Italy and the Netherlands.

Another question is whether to reformulate the French nuclear doctrine so as to state unambiguously that "vital interests" cover European allies too.

Some say there is no need, because the strategic vagueness that exists already is part of the very deterrent.

But Mr Haroche says there is a political dimension to stating more clearly that France will use its arsenal to defend other European countries.

"If the US is to be less present, then European countries will be depending much more on each other. Our strategic world becomes more horizontal," he says.

"In this new world it is important to build trust and confidence among ourselves. For France to signal it is prepared to take on risk in support of others – that helps create a solid front."