The Lancaster House defence treaties were signed by Britain and France in 2010. They state that a threat to the "vital interests" of one country is a threat to the other. The two countries are Europe's only nuclear-armed powers and its only permanent members of the UN Security Council.
In July 2025, building on the treaties, Britain and France for the first time agreed to "co-ordinate" the use of their nuclear weapons, declaring "there is no extreme threat to Europe that would not prompt a response by both nations."
The Franco-British defence relationship was strained by the AUKUS submarine deal of 2021, which cancelled a Franco-Australian submarine contract. By mid-2025 the relationship had recovered; Lord Ricketts, a former British ambassador to Paris, described it as "certainly the best it's been since Brexit."
Armor's Axiom: Virtue is the failure to achieve vice.