Explainer-How realistic is France's offer to extend its nuclear umbrella?

By John Irish

PARIS (Reuters) - President Emmanuel Macron has said he will launch a strategic dialogue on extending the protection offered by France's nuclear arsenal to its European partners.

Europe's primary nuclear deterrence comes from the United States and is a decades-old symbol of trans-Atlantic solidarity.

France has a much smaller nuclear arsenal than its NATO ally but Moscow has responded to Macron's remarks by saying his nuclear rhetoric poses a threat to Russia.

WHY IS FRENCH NUCLEAR DETERRENCE BEING DISCUSSED NOW?

U.S. President Donald Trump's questioning of burden-sharing in NATO and his overtures to Russia as he tries to end the war in Ukraine have led to existential questions in Europe about the trans-Atlantic relationship and about Washington's commitment to helping defend its European allies.

Friedrich Merz, who is likely to become Germany's chancellor, has said he fears NATO may not remain in its "current form" by June and advocated a closer look at French and British nuclear deterrence.

The U.S. has nuclear arms in Europe and tens of thousands of troops deployed in bases across the continent. It provides some military capabilities that Europe cannot match, from mid-air refuelling to its nuclear umbrella.

Nuclear deterrence is enshrined in NATO’s Article V collective defence pledge, and has for decades been backed by U.S. politicians. But the assurances that nuclear weapons are a core part of the alliance's deterrence strategy are not legally binding.

HOW BIG IS FRANCE'S NUCLEAR ARSENAL?

France is the European Union's only nuclear power following Britain's departure from the bloc and Paris takes pride in its independent nuclear deterrent.

France spends about 5.6 billion euros ($6.04 billion) annually on maintaining its stockpile of 290 submarine- and air-launched nuclear weapons, the world's fourth largest. It spends about 15% of its annual defence budget on modernising its nuclear capabilities.

The U.S. and Russia both have more than 5,000 nuclear warheads, according to the Federation of American Scientists. The U.S. has 1,700 warheads deployed globally.

Britain describes its nuclear programme as "operationally independent", but sources missile technology from the U.S. and depends on the U.S. for acquisition and maintenance support.

WHAT DOES FRANCE'S NUCLEAR DOCTRINE SAY?

The doctrine is defensive, designed to deter potential aggressors by maintaining a credible nuclear threat and to protect France's vital interests. Macron has said those interests have a "European dimension" without giving details.

A policy of "strict sufficiency" means the French nuclear arsenal is kept at the lowest level possible and is to be used only in extreme circumstances of self-defence.

Macron, like earlier French presidents, has refused to place the French nuclear deterrent under EU or NATO command. He maintained that position in Wednesday's address.

HOW DO THE FRENCH AND U.S. NUCLEAR UMBRELLAS COMPARE?

Unlike the U.S., France does not offer an extended commitment to deter or respond to nuclear or non-nuclear threats on allies to defend them.

France has only a fraction of the number of airborne nuclear weapons the U.S. can provide. Any upgrade of its systems would be costly and require significant logistical and operational changes at a time when governments are already stretched financially. It could take France up to a decade to increase its arsenal by only 100 warheads, experts say.

The U.S. deploys its nuclear arsenal across Europe. France's nuclear doctrine has prohibited the stationing of atomic weapons outside France.

"In the current situation, the French and British nuclear forces are a complement to U.S. extended deterrence, but they would not constitute a viable solution in the event of an abrupt withdrawal of U.S. nuclear forces," the Washington-based Centre for Strategic & International Studies said.

"France and the United Kingdom do not provide an extended deterrence guarantee the way the United States does in NATO."

WHAT COULD HAPPEN NEXT?

There is no suggestion of replacing the U.S. nuclear infrastructure like-for-like.

Allies could be integrated into French nuclear exercises and training through the provision of air escorts and by the development of supply and logistical support, aiming to create a degree of interoperability, analysts say.

France possesses Rafale warplanes capable of carrying nuclear missiles. These planes could be deployed across Europe

with the aim of creating "strategic ambiguity" for Russia.

The issue of whether EU partners could contribute to the cost of maintaining or upgrading the French deterrence could be one option to be discussed, experts said.

($1 = 0.9265 euros)

(Reporting by John Irish, Editing by Richard Lough and Timothy Heritage)