The two countries have built the deepest intelligence alliance in the history of the Western world. It has been gradually forged through the common fight against Hitler’s fascism, Soviet communism, and the Global War on Terror. In the aftermath of World War II, the two nations agreed to collect and share sensitive signals intelligence as part of the 1946 United Kingdom–United States of America (UKUSA) Agreement. At the time, the United States leaned heavily on well-established British intelligence capabilities from across the vast British Overseas Territories and Commonwealth to surveil Soviet activities. This cooperation laid the foundation for the Five Eyes intelligence-sharing alliance between the United Kingdom, United States, Canada, Australia, and New Zealand, which was formalized in 1956.
The Five Eyes has since developed into the world’s most significant intelligence-sharing alliance, rooted in extensive coordination and sharing of both raw and finished intelligence. In many ways, it is considered the gold standard of intelligence alliances and represents the backbone of the special relationship. For example, the United Kingdom benefited immensely from Five Eyes intelligence sharing during the Cold War, when U.S. monitoring of Soviet submarines proved vital to coordinated defensive efforts in the North Atlantic and North Sea. In return, British listening posts in its former imperial territories provided crucial signals intelligence to the United States on Soviet activities in the Middle East.
Functionally, the Five Eyes operates across a wide spectrum of domains and geographies. Allied intelligence organizations monitor maritime traffic through strategic areas, surveil ballistic missile tests and satellite deployments, and coordinate on collective offensive capabilities in the cyber domain. Moreover, numerous agreements exist in discrete, highly sensitive areas such as chemical and biological defense, which entail extensive technical cooperation at the working level.
Nuclear Technology
Cooperation on nuclear technology is another core tenet of the special relationship with roots in the early Cold War era. The 1958 Mutual Defense Agreement formed the basis for closer ties and allowed the UK nuclear arsenal to incorporate U.S. technological advancements. Since 1962, the United Kingdom has acquired U.S. nuclear weapons delivery technology while producing its own submarines and warheads. Furthermore, its Trident system, which includes four Vanguard-class nuclear-powered and nuclear-armed submarines, relies on maintenance by U.S. company Lockheed Martin, and the United Kingdom procures the aeroshells required to produce warheads from the United States. The Trident system is thus closely linked to U.S. systems, though they ostensibly operate independently.
The Mutual Defense Agreement was extended indefinitely in 2024 and includes provisions that make it difficult for either party to terminate the arrangement. Locking in the bilateral nuclear-sharing arrangement on a more or less permanent footing has provided some important assurances of continued U.S.-UK cooperation, irrespective of domestic political changes. Moreover, the extension provides some stability and budget certainty at a time when London is modernizing its stockpile of nuclear weapons and delivery systems, replacing the aging Vanguard-class submarines with a new Dreadnought-class platform that will enter into service in the early 2030s. Budget overruns and delays related to this modernization drive a significant risk, as the Dreadnought program has already led to a 41 percent increase in projected costs for the Royal Navy as of December 2023.
Defense Industrial Base
The 2021 Defence and Security Industrial Strategy (DSIS) reoriented strategic thinking about the British defense industrial base. It emphasized building a resilient defense industry without reliance on foreign suppliers—a marked shift from past governments’ tendency to outsource defense production to reduce costs. Russia’s full-scale invasion of Ukraine, the Covid-19 pandemic, and Brexit have shifted attention across the political spectrum toward prioritizing maintenance of a sustainable UK industrial base. The SDR recognizes the UK defense industry as a vital engine for growth, both in its security ambitions and long-term economic outlook. In FY 2023, the Ministry of Defence invested nearly $40 billion with UK industry, supporting 440,000 jobs. Defense industrial relationships, the most significant being with the United States, contributed to $19.7 billion in overall British defense exports.
The United States remains the United Kingdom’s closest defense partner. Its Foreign Military Sales portfolio with the United States is valued at more than $18.6 billion in joint projects, particularly the F-35 program, demonstrating the high level of interoperability and industrial integration that characterizes the relationship. As a tier-one partner on the program, the United Kingdom contributes critical components to the F-35 supply chain, with approximately 15 percent of the value of each aircraft attributable to the United Kingdom. This includes the Martin-Baker ejection seat, Cobham refueling probes, and BAE Systems tail sections. As a result, the F-35 plays a significant role in the British labor market. The Royal Air Force’s XVII Test and Evaluation Squadron, housed at Edwards Air Base in California, operates primarily to bring F-35 aircraft into service. The partnership, focused on integrated procurement and maintenance, has been a foundation of economic competitiveness in defense for both partners.
The 2021 Australia–United Kingdom–United States (AUKUS) agreement enhances the unique bond between the three treaty allies and ensures cooperation in the Indo-Pacific. The last time a similar agreement was reached was in 1958 with the U.S.-UK Mutual Defense Agreement. AUKUS aims to promote a “free and open Indo-Pacific that is peaceful, secure, and stable,” as reflected in two main pillars. Pillar One focuses on delivery of nuclear-powered attack submarine capabilities, including highly sensitive nuclear propulsion technology that Washington has historically held sacred. By 2040, the SSN-AUKUS will replace the United Kingdom’s Astute-class attack submarine as one of the world’s most advanced submarines. Pillar Two is broader and more complex, with a clear focus on enhancing joint capabilities and interoperability across several domains, including “cyber capabilities, artificial intelligence, quantum technologies, and additional undersea capabilities,” as well as electronic warfare, hypersonic technologies, and enhanced information-sharing protocols. Its implementation will demand investment in both the U.S. and UK defense industrial bases, relying on joint innovation and shared research and development. So far, however, there has been limited progress made on implementing Pillar Two. Looking forward, the success of AUKUS will not solely depend on adequate financing; just as critical will be whether the United Kingdom has the infrastructure to produce innovative technology at scale.
At the same time, regulatory reforms, particularly the International Traffic in Arms Regulations (ITAR) exemptions announced in September 2024, have attempted to ease export controls and other regulatory barriers that had limited U.S.-UK defense industrial integration. Removing such barriers could accelerate procurement opportunities and access to both countries’ defense technology sectors but also has the potential to form a trilateral common market on defense that incentivizes strengthening this relationship. Such trilateral defense cooperation could enhance battlefield integration and technological innovation across theaters. Consequently, the AUKUS partnership offers a strategic counterweight to China and reinforces allied unity in a rapidly evolving landscape of geopolitical competition.
Basing
The United States, which has maintained military bases in the United Kingdom since 1942, currently deploys approximately 10,000 military personnel on British soil, not including a significant number of contractors and civilian employees. In turn, approximately 750 military and civilian personnel with the Ministry of Defence train at U.S. military bases across 30 states.
Each nation also benefits from the other’s global footprint of military bases. The United States has utilized military infrastructure in overseas UK territories, such as Ascension Island in the Atlantic Ocean and Diego Garcia in the Indian Ocean, which has housed a shared U.S.-UK military base since the 1970s. The United Kingdom finalized an agreement in May 2025 on the future of the Diego Garcia archipelago that would hand territory over to Mauritius but allow the Diego Garcia base to continue operation on a 99-year lease. The negotiations, while met with criticism by defense hawks in both the United Kingdom and United States, emphasized the territory’s strategic importance for power projection of two uniquely entangled allies by focusing on maintaining joint operational forces and power projection in the region. Importantly, the United States was not an official party in the agreement. However, the final agreement was met with high praise by the Trump administration as a vital success for U.S. national security interests in the Indian Ocean and beyond.
Special Operations Forces
Interactions between the U.S. and UK militaries have historically taken place through a patchwork of relationships between specialized communities. Although there are no formal frameworks governing cooperation, the relationship between the countries’ special operations forces is particularly significant in this regard. From a U.S. perspective, what the UK forces have not been able to deliver in terms of quantity has been compensated by their quality and interoperability with U.S. units. For example, the two countries have deployed special forces together as part of the Joint Special Operations Command Task Force in the Iraq War, a temporary grouping often assigned to engage high-value targets. U.S. and UK special operations units also regularly train together, both on a bilateral level and through NATO.
The United Kingdom’s Current Strategic Dilemma The United Kingdom and Europe After Brexit
In 2016, British voters opted to leave the European Union in a referendum that altered the economic and political trajectory of the United Kingdom. In the ensuing nine years, the United country has become more estranged from the continent and has experienced economic stagnation.
Following the Brexit vote, UK strategy sought to turn away from Europe and toward the rest of the world. Prime Minister Teresa May declared that the post-Brexit era of “Global Britain” was beginning. After officially departing the European Union on December 31, 2020, the UK government conducted an “Integrated Review of Security, Defence, Development and Foreign Policy,” in essence a national security strategy, that paid almost no attention to Europe. Instead, “Global Britain” would aggressively seek new trading agreements around the world. For instance, the United Kingdom signed onto the Comprehensive and Progressive Agreement for Trans-Pacific Partnership, a trade agreement involving 11 countries in the Indo-Pacific and the Americas, in July 2023.
However, instead of accelerating growth by removing the “shackles” of EU regulation, Brexit has created structural barriers that have exacerbated the United Kingdom’s economic challenges. In particular, it has made it more cumbersome to do business with the European Union. Given the European Union has around 450 million people and is an economy roughly equivalent in size to the United States or China in terms of purchasing power, Brexit erected barriers to an enormous neighboring market in which British producers had been highly enmeshed.
Separately, following the Global Financial Crisis of 2007–08, London enacted deep austerity measures. A lack of economic investment, together with the uncertainty and friction from Brexit, has led to sluggish growth and reduced investment. Covid-19 also contributed to high inflation, which was then exacerbated by an energy crisis following the sabotage of the Nord Stream pipelines, leaving the United Kingdom with some of the highest energy prices in Europe. The United Kingdom was the only member of the G7 yet to restore pre-pandemic levels of GDP by 2023, with foreign direct investment inflows falling 37 percent for six years prior. It then slipped into a technical recession in the final quarter of 2023.