This bilateral treaty established the deepest nuclear cooperation agreement between any two nations, underpinning the UK's nuclear deterrent for decades.
Key Facts
- Date signed
- 3 July 1958
- UK plutonium transferred to US
- 5.4 tonnes (1960–1979)
- HEU received from US
- 7.5 tonnes
- Tritium received from US
- 6.7 kilograms
- Times amended/renewed
- 10 times
- Extended indefinitely
- 14 November 2024
By the Numbers
Location
Cause → Event → Consequence
The Soviet Union's launch of Sputnik in October 1957 alarmed the American public, while Britain had demonstrated thermonuclear capability through Operation Grapple in November 1957. These developments created mutual strategic incentive for the US and UK to formalise deep nuclear cooperation.
Signed on 3 July 1958, the US–UK Mutual Defence Agreement permitted the exchange of nuclear materials, technology, and information between the two countries. It included the sale of a complete nuclear submarine propulsion plant and a ten-year supply of enriched uranium to the United Kingdom.
The UK became significantly dependent on the US for nuclear weapons technology, while gaining access to American missiles for its Polaris and Trident programmes. The treaty paved the way for the Polaris Sales Agreement and shaped British nuclear strategy well into the twenty-first century, with the agreement extended indefinitely in 2024.
Political Outcome
The treaty was signed and has been renewed ten times; on 14 November 2024 it was extended indefinitely, cementing long-term US–UK nuclear cooperation.
UK pursued an independent nuclear programme with limited US cooperation
UK became closely reliant on US nuclear technology, materials, and missile systems for its deterrent