Europe is asking a question it thought it had settled in 1949. If the American nuclear umbrella is no longer reliable, can France and Britain hold it up alone? The question is no longer theoretical. It has moved from the back rooms of Brussels into public debate. And the answers coming back are not comforting.
In March, Emmanuel Macron stood at the Ile Longue submarine base in Brittany and delivered a speech that broke a half-century of doctrinal silence. He announced that France’s nuclear deterrent, long understood as a strict national guarantee, could be extended to cover European partners. He called it “dissuasion avancee,” forward deterrence. The message was clear: France is prepared to defend the continent with its own arsenal if the United States steps back.
The problem is one of numbers and trust. France operates approximately 300 warheads, delivered by four Triomphant-class submarines and a small air force component. The British Trident system, based on four Vanguard-class submarines, adds roughly 225 warheads at the deployed operational level. Combined, the two countries field about 525 warheads. Russia fields roughly 5,580. The disparity is not trivial, but neither is it decisive. Deterrence does not require parity. It requires the credible threat of unacceptable retaliation.
Credibility is the harder variable.
France’s nuclear doctrine vests final launch authority in one person: the president of the Republic. No ally has a seat at that table. Macron’s speech gestured at burden-sharing, joint exercises, shared intelligence, consultation mechanisms, but he did not propose a European finger on the trigger. No French president could. The Constitution does not permit it. European partners would be asked to accept a guarantee they cannot fully verify and cannot execute.
The British case is different and in some ways more fragile. The UK’s Trident missiles are leased from the United States. The warheads are British, but the guidance systems, aeroshells, and test infrastructure all depend on American cooperation. Washington does not have a veto on British launch, but it does have a choke point on maintenance. A UK that diverged from US strategic priorities would find its nuclear capability slowly degrading. The European Nuclear Study Group, a body of analysts and former officials, has described this as a structural vulnerability.
The study group’s 2025 warning was blunt. It identified a growing risk of “nuclear coercion” by Russia, the use of nuclear threats to fracture the alliance, and described US extended deterrence as “fragile.” That fragility is now a working assumption in European defense planning.
Germany’s Chancellor Friedrich Merz has publicly floated nuclear cooperation with France and the UK. Sweden and Poland are exploring bilateral arrangements with France. These are not NATO procedures; they are bilateral defense diplomacy, conducted outside the alliance framework. They signal a recognition that the alliance itself may not be the vehicle for the solution.
But the hard truth is this: no European nuclear deterrent can replace the American one in the near term. The size of the combined French-British arsenal is sufficient for existential national defense. It may not be sufficient for extended deterrence across a continent of 450 million people stretching from Lisbon to Tallinn. The logic of extended deterrence requires the defender to be willing to absorb a first strike on behalf of another country. That was always the question at the heart of the Cold War: would Washington risk Chicago for Hamburg? The answer under every president was yes, because the alliance was a structure of shared fate, not a ledger of capabilities.
Europe has no equivalent structure for shared nuclear fate. It has two independent nuclear powers with separate command chains, different doctrines, and no joint decision-making mechanism. France and the UK do not even hold joint nuclear planning exercises. They do not share target sets. They do not have a combined strategic directive.
That may change. The European Nuclear Study Group has proposed a new force posture concept: a European nuclear consultation framework with pre-authorized release procedures, joint targeting, and shared operational planning. It would not be a European nuclear arsenal. It would be a European nuclear consensus, a set of agreements that would make retaliation automatic enough to be credible.
The obstacle is political, not technical. France would have to accept constraints on its sovereign decision-making. The UK would have to decouple its deterrent from American supply chains. Germany, Sweden, and Poland would have to accept that their security depends on weapons they do not control. These are not impossible steps. They are steps no European government has yet been willing to take.
The window for taking them may be smaller than anyone wants to admit.