Why U.S.-U.K. Nuclear Cooperation is More Important than Ever
With the risk of nuclear escalation at its most significant since the Cold War, the 1958 U.S.-U.K. Mutual Defense Agreement (MDA) will expire at the end of this year. The treaty, specifically its Article III, has long authorized the special sale of nuclear materials to Britain, forming not just the cornerstone of Anglo-American nuclear sharing but perhaps the entire ‘special relationship.’
In this security environment, the upcoming re-ratification and ten-year extension of the MDA will be the most critical in its history. The terms of the treaty itself are unlikely to change significantly. However, the renewal will take both countries through a defining period of modernization across their entire arsenals as the realities of competing with two near-peer nuclear adversaries are fully realized.
What has previously been a secretive routine procedure should be reframed under these conditions to strengthen the NATO deterrent and advance vital dual progress on nonproliferation and disarmament. Publicly doubling down on the enduring U.S.-U.K. nuclear relationship can reassure allies concerned about Washington’s long-term commitment to extended deterrence and London’s deterrence capabilities. It will also inject a long-needed democratic process into the treaty.
“In the National Interest”
Signed in 1958, the MDA allows the United Kingdom and the United States to “share classified nuclear information, nuclear technology, and scientific knowledge in relation to each countries’ respective nuclear weapons programs.” Alongside the 1963 Polaris Sales Agreement (PSA), which authorized the sale of the United States’ Polaris submarine-launched ballistic missile (SLBM) system and, subsequently, the Trident SLBM system to the United Kingdom, it forms the core of the two countries’ defense cooperation and London’s strategic deterrent.
The MDA itself is not time-limited. However, per the terms of the original agreement, Article III expires after ten years and requires renewal. This article is considered the most important part of the treaty, allowing for “the transfer of special nuclear materials and non-nuclear components,” including one submarine nuclear propulsion plant and the plutonium or highly enriched uranium that runs it.
The pact was last renewed in 2014 when current nuclear modernization programs in both countries were still nascent. The previous renewal featured a rare amendment to Article III, which authorized but did not require Washington to provide more than one propulsion plant. This only deepened the appearance of British reliance on the United States to maintain its deterrent.
The full extent of cooperation authorized under the MDA remains unknown due to classification. It likely covers joint research, including site visits, as well as some degree of collaboration on a brand new nuclear warhead, the W93, which compliments the shared use of the Trident II D5 missile per the terms of the PSA.
Upon renewal in 2014, then-President Barack Obama told Congress that “it is in the United States’ national interest to continue to assist the United Kingdom in maintaining a credible nuclear deterrent.” The convergence of several generational challenges makes the stakes for this year’s renewal even greater: the modernization of each nation’s nuclear arsenal, new threats to traditional deterrence, growing disarmament advocacy, and respective domestic political change.
Challenge Number One: Nuclear Modernization
Both the United States and the United Kingdom are undergoing complete modernization of their nuclear weapons programs across the coming decades. For the former, this includes replacing the delivery vehicles of each leg of the nuclear triad with new models—namely, the Columbia-class ballistic missile submarines, the B-21 strategic bomber fleet, and the Sentinel intercontinental ballistic missiles. The United Kingdom is also replacing its four Vanguard-class ballistic missile submarines with the new Dreadnought-class.
Since London only fields a sea-based deterrent, U.S.-U.K. cooperation has been limited to the Navy. While the programs are officially sovereign, limited collaboration is inevitable on the W93 nuclear warhead and the Mk 7 aeroshell re-entry body that will carry it, given that both countries share a common pool of Trident II D5 SLBMs. The W93 will be Washington’s first brand-new nuclear weapon since the Cold War.
By the time the MDA renews again in 2034, many of these programs will be well underway or even completed, making this year a pivotal moment to demonstrate lasting commitment to the relationship. The upcoming renewal of the agreement will thus mark a crucial period in both countries’ modernization efforts, which are intrinsically linked by the Trident missiles.
Challenge Number Two: Nuclear Deterrence
The current global security environment, marked by Russia’s war in Ukraine, conflict in the Middle East, and ongoing fears about a Chinese invasion of Taiwan, means the West must simultaneously assess the credibility of its own nuclear deterrence, as well as the fragility of the doctrine at large. For the first time, Washington and its allies confront two near-peer nuclear adversaries: Russia and China.
Cooperation in the context of the MDA may soon deepen with the deployment of U.S. nuclear weapons on British soil for the first time since 2008. Plans to base B61-12 gravity bombs at RAF Lakenheath, deliverable by NATO dual-capable F-35A Lightning II aircraft, would signify a significant commitment to U.S. extended deterrence in Europe.
That assurance from Washington is further welcomed by recent concerns about the capability of London’s sovereign deterrent. In January, during a routine Demonstration and Shakedown Operation of a newly refitted Vanguard-class submarine, the Royal Navy encountered a second successive failed launch of a test Trident II missile. While British officials were quick to downplay the failure, there have been heightened concerns about the U.K.’s deterrent capabilities in a possible wartime situation.
These factors make this year’s MDA renewal even more critical. While not its core function, it can strengthen Western deterrence by publicly reinforcing the unity of NATO’s nuclear backbone and assuaging doubts about Britain going it alone.
This task is even more critical considering allied concerns about a possible second Donald Trump administration. The former president’s recent remarks cast doubt on long-term U.S. commitments to NATO, including its extended nuclear deterrence if elected for a second term. Those comments prompted European allies to look to the United Kingdom as the only other NATO member to commit its nuclear stockpile to the alliance’s deterrent.
Challenge Number Three: Nonproliferation, Disarmament, and Risk Management
The 2014 MDA renewal received fierce criticism for its impact on nonproliferation and disarmament, particularly in London. Given the comparative growth of the disarmament movement since 2014, marked by the Treaty on the Prohibition of Nuclear Weapons, this year’s renewal will no doubt face similar opposition.
Critics of the MDA point to its conflict with the widely endorsed Treaty on the Nonproliferation of Nuclear Weapons (NPT). The NPT commits State Parties, including the United States and the United Kingdom, to both prevent the transfer of nuclear weapons (Article I) and pursue good faith efforts toward nuclear disarmament (Article VI)—both of which appear to run counter to the MDA.
To maintain enduring legitimacy and work more meaningfully towards nonproliferation and disarmament, the framing of this year’s MDA renewal should be more inclusive and open to discussion, starting with a debate in the British parliament. This will provide a platform for both governments to highlight how the agreement advances transatlantic security by bolstering deterrent capabilities, and it is consistent with efforts to prevent nonproliferation and work toward disarmament.
Challenge Number Four: Domestic Political Factors
Lastly, significant political upheaval in Washington and London, where elections are expected in the months around the treaty’s re-signing, add to the moment’s significance. While these will not affect the MDA in the short term, new administrations in both countries could reshape the ten-year proceeding, especially on U.S. commitments to NATO. Significant elections may even also overshadow the renewal process altogether.
A second Trump administration will likely force the United Kingdom to take on a more assertive role in the relationship between the two countries and NATO. While it’s unlikely that the MDA would be in the former president’s crosshairs, the timing allows both parties to lock in the renewal before any change in governance in the White House.
The 2024 MDA Renewal: A Path Forward
Therefore, this year’s renewal of the MDA comes amidst a plethora of potentially definitive challenges for both countries. Though significant alterations to the agreement are unlikely, the landmark moment can be utilized to help navigate them. The renewal process of the treaty should not remain secretive as in previous years.
While there are obvious limitations to releasing some sensitive information, celebrating the continuation of the MDA and reinforcing the inherent benefits to both countries—and to the wider NATO alliance—can strengthen resolve and credibility in a degraded security environment. Doing so is especially important in the context of Russia’s war in Ukraine, as well as domestic concerns about deterrence caused by incremental progress on modernization and British test failures.
The entire renewal process is typically carried out in great secrecy and without debate, particularly in London, which also harms the agreement’s credibility as a function of two leading democracies. Parliamentary scrutiny of national security issues, particularly on the nuclear deterrent, is often restricted in the United Kingdom, which has only exacerbated accusations that the MDA does not comply with Britain’s agreements under the NPT.
Allowing parliament to debate—and the option to veto, if they see fit—the upcoming renewal of the MDA can bolster the agreement’s legitimacy, foster public understanding and trust, and provide the government an opportunity to defend its NPT commitments. In previous years, the U.S. Congress has at least had that opportunity. At a time when the risk of nuclear escalation is growing, taking these steps would also serve as a welcome moral recommitment to long-term peace by two leading nuclear powers without undermining critical security collaboration.
Views expressed are the author’s own and do not represent the views of GSSR, Georgetown University, or any other entity. Image Credit: U.S. Department of Defense