In July 2025, the United Kingdom and France signed the “Northwood Declaration”, announced by UK Prime Minister Keir Starmer and French President Emmanuel Macron. The declaration commits the two nuclear-armed states to deepen cooperation on defence and security, and to enhance coordination between their respective ‘independent’ nuclear weapons systems. This accord is the latest manifestation of a long tradition of defence collaboration upheld by a succession of UK Prime Ministers and French Presidents, reaffirming both countries’ commitment to mutual defence and bilateral security ties.
The origins of this cooperative spirit can be traced to the Treaty of Dunkirk (1947), signed in the aftermath of the Second World War – prior to the establishment of American extended deterrence to Europe – as a mutual defence pact against a potential revanchist Germany (this treaty was a precursor to the eventual creation of NATO).
In the decades following both nations’ acquisition of nuclear weapons, additional agreements have sought to deepen this partnership, including in the nuclear domain. These include the Chequers Declaration of 1995, agreed upon by Prime Minister John Major and President Jacques Chirac, and the Lancaster House Treaties of 2010, concluded between Prime Minister David Cameron and President Nicolas Sarkozy. This included a treaty sparking collaboration on warhead research in joint facilities through the TEUTATES project. The Northwood Declaration builds upon and updates these earlier frameworks but has been distinguished as the first instance in which the UK and France will “coordinate” their nuclear weapons systems, though there is broad speculation as to how this coordination will (and should) physically manifest.
Despite this long-term pattern of defence cooperation, the relationship, particularly in nuclear matters, has not been without strain, as shown by the brief diplomatic fallout following the AUKUS submarine deal, in which Australia signed a pact with the UK and U.S. to manufacture nuclear-powered, conventionally armed submarines, which cancelled a prior agreement between France and Australia to manufacture conventional submarines.
On a different note, the Northwood Declaration is consistent with the commitment from the UK’s latest Strategic Defence Review (SDR) to build on the Lancaster House Treaties. Additionally, and like the rationale behind the UK’s recent participation in NATO’s dual-capable aircraft nuclear mission (similarly presaged by the SDR), the declaration reflects an effort to strengthen European regional deterrence amid growing concerns about the reliability of American extended deterrence. Therefore, and in contrast to the Treaty of Dunkirk, which was concluded prior to NATO’s formation, the Northwood Declaration can serve as a hedge against the potential weakening of a central pillar of the alliance: the U.S. security guarantee. Despite this, to the extent that the UK’s policy of Continuous-At-Sea-Deterrence (CASD) is perceived as central to ensuring European security, any challenges to CASD, such as the aging submarine fleet, could undermine the long-term credibility of these nuclear declarations.