1958 US–UK Mutual Defence Agreement

The 1958 US–UK Mutual Defence Agreement, or UK–US Mutual Defence Agreement, is a bilateral treaty between the United States and the United Kingdom on nuclear weapons cooperation. The treaty's full name is Agreement between the Government of the United States of America and the Government of the United Kingdom of Great Britain and Northern Ireland for Cooperation on the uses of Atomic Energy for Mutual Defense Purposes.

It was signed after the UK successfully tested its first hydrogen bomb during Operation Grapple. While the US has nuclear cooperation agreements with other countries, including France and some NATO countries, this agreement is by far the most comprehensive.

The treaty is renewed every ten years, most recently extending the treaty to 31 December 2024.

The agreement enables the US and the UK to exchange classified information with the objective of improving each party's "atomic weapon design, development, and fabrication capability".

This includes development of defence plans; training personnel in the use and defence against nuclear weapons; evaluation of enemy capabilities; development of nuclear delivery systems; and research, development and design of military reactors. The agreement also provides for the transfer of special nuclear material (e.g., plutonium, highly enriched uranium, tritium), components, and equipment between the two countries, and the transfer of "non-nuclear parts of atomic weapons" to the UK.

The agreement also covered the export of one complete US submarine nuclear propulsion plant and its enriched uranium fuel which was installed in the UK's first nuclear-powered submarine, HMS Dreadnought.

The UK was able to carry out underground nuclear tests at the US Nevada Test Site, the first taking place on 1 March 1962, following this agreement.

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