The Mutual Defense Agreement

For 65 years, the treaty between the United States and the United Kingdom has allowed the two countries to exchange nuclear materials, technology, and information.

By Micaela Hester | April 28, 2023

Mutual Defense Feature Opt
Scientists use AWE’s Orion laser facility to research high-energy density plasma physics phenomena, which occur at the heart of nuclear explosions and inside stars. Atomic Weapons Establishment

Nuclear collaboration between the United States and the United Kingdom dates back to World War II. Not only were the two countries allies, but British scientists contributed to the first atomic weapons developed during the Manhattan Project.

After the war, political and scientific leaders on both sides of the Atlantic wrestled with how to manage nuclear capabilities and information exchange across nations. Following the restrictive United States Atomic Energy Act of 1946, the two countries were unable to share nuclear information. They pursued separate, parallel nuclear capabilities over the next several years, with the United Kingdom successfully testing its first atomic weapon in 1952. After years of discussions, negotiations, and the continued development of the countries’ respective nuclear programs, the U.S.–U.K. Mutual Defense Agreement (MDA) was signed on July 3, 1958. The bilateral treaty (officially titled the Agreement between the Government of the United Kingdom of Great Britain and Northern Ireland and the Government of the United States of America for Cooperation on the Uses of Atomic Energy for Mutual Defense Purposes) provides for the exchange of defense information relevant to nuclear weapons, naval nuclear propulsion, and nuclear threat reduction.

After the MDA was signed, U.K. and U.S. nuclear collaborations flourished and continue to thrive today, nearly 65 years later. Exchanges through the MDA benefit both the United States and the United Kingdom by advancing each country’s understanding of the safety, security, and effectiveness of their respective nuclear weapons stockpiles.

The current scientific exchange is primarily conducted among Los Alamos, Livermore, and Sandia national laboratories in the United States and the Atomic Weapons Establishment (AWE) in the United Kingdom. As both countries continue to modernize their nuclear weapons, one area of recent collaboration has been the W93—a potential new warhead being designed at Los Alamos that would be deployed on submarines.

“The W93 continues our strong partnership with the United Kingdom,” explained Jill Hruby, head of the National Nuclear Security Administration, at the 2023 Nuclear Deterrence Summit. “Our efforts in this area constitute a separate but parallel effort to the UK’s replacement warhead project for its submarine-based missiles.” Through this work, the two independent but allied nuclear powers will continue to support one another’s national security as well as the defense needs of the North Atlantic Treaty Organization.

The MDA will be up for renewal in 2024.