Writing for the Daily Mail in 2023, Former British Prime Minister Boris Johnson sardonically argued , “The Americans don't give us crucial nuclear secrets because they love little old England. They don’t share intelligence because they adore our quaint accents. We have a great and indispensable relationship because we have important things to offer – including Diego Garcia.”

In March, during the ongoing war, Iran launched two missiles at Diego Garcia, which houses a joint UK-American base in the Indian Ocean. While both missiles missed their target, the attack provided the first operational demonstration of Iran’s intermediate-range ballistic missile capability. It also offered a reminder that security in the Indian Ocean is inextricably linked with the larger Middle East. Or, as a senior American naval official put it decades earlier, the Indian Ocean was the “backdoor” to the Middle East and Russia and China’s “soft underbelly.”

To understand why Iran targeted an island 2,500 miles from its shores, turn to Samuel Bashfield’s new and well-drawn history of the U.S. and U.K.’s Cold War adventures in the Indian Ocean, entitled simply “The British Indian Ocean Territory.” As Bashfield effectively demonstrates, the island is central to America’s maritime posture in the Indian Ocean. It has been so since the Second World War.

Today, Diego Garcia is crucial for naval logistics, air power facilitation, military supply prepositioning, and intelligence gathering. It is also a key node for operations across the Indian Ocean, Middle East, South Asia, and Africa. In Bashfield’s words, it is “a coveted lily pad.” For Washington’s strategy, Diego Garcia is to the Indian Ocean as Guam is to the Pacific.

For this work, Bashfield has accessed materials from relevant archives in the U.S., U.K., and Australia. While some documents still remain confidential due to government regulations, the ones used to piece this volume together show a clear pattern.

Bashfield, who holds a doctorate in maritime history and national security policy, begins his book by recounting the importance of Diego Garcia to the Allied war effort during the Second World War. Back then, Japan’s rapid advances in Southeast Asia threatened British Singapore. In 1941, the British decided to use Diego Garcia as a naval logistics base to cover the Indian Ocean.

However, it was only during the early Cold War, amid rapid decolonization in parts of Asia and Africa, that British and American military planners grasped the value of Diego Garcia and the entire Chagos Archipelago’s strategic real estate. Successive policy measures like the Indian Ocean Task Force (1959), America’s Strategic Island Concept (1960), and Britain’s Island Strategy (1963) outlined this thinking.

Bashfield goes into rich detail about U.S. and British negotiations over the Chagos Islands and Diego Garcia and their “detachment” from colonial Mauritius and Seychelles. He further shows how, during the late 1960s, London and Washington wanted to entrench each other’s position in the Indian Ocean. Couched within this larger history of the British Indian Ocean Territory, there is also an interesting story about the lesser-known Aldabra Island, which captivated British planners as a potential military facility.

Over the years, the United States and Britain negotiated three quid pro quo deals that linked expanded American access to Diego Garcia with the British nuclear deterrent. In exchange for an expanded American presence on Diego Garcia, Britain secured American assistance in three successive submarine-launched ballistic missile programs: Polaris, Super Antelope and Trident. These capabilities allowed London to stay comfortably in the nuclear club, possess a second strike capability and enhance undersea deterrence. In this manner, Bashfield’s work on maritime history also makes useful forays into nuclear history. As then-British Prime Minister Margaret Thatcher said, her decisions on nuclear technology came with “the greatest long-term consequences for Britain.”

In the latter half of his book, Bashfield deals with the “coming of age” of Diego Garcia. Given the expansion of American military infrastructure over decades, the author argues that the British Indian Ocean Territory has become a “de facto American territory,” despite occasional hand-wringing about sovereignty from London. Diego Garcia’s strategic value was burnished when it was used as a key logistics node and airbase during Operations Desert Shield and Desert Storm, after Saddam Hussein’s unprovoked invasion of Kuwait in 1990.

One aspect I would have liked to know more about was the international scene, especially the view of other major actors, while the crucial negotiations between British and American officials proceeded. To be sure, the author touches upon the role of the Soviet Union, India, and some other littoral states in the context of these negotiations. However, these are not fully fleshed out.

In our time, Britain is caught in a muddle trying to dissolve the British Indian Ocean Territory and help ease Mauritian sovereignty over the Chagos archipelago. In his customary style, President Donald Trump has induced unpredictability into this dynamic. He enjoys haranguing British officials for signing a “stupid” deal to return the island chain to Mauritius. Secretary of State Marco Rubio, on the other hand, has called the deal “historic” in his remarks and has supported both Britain and Mauritius in their negotiations.

In exchange for Mauritian sovereignty over the entire Chagos archipelago, the United Kingdom can operate the Diego Garcia base – through the U.S. – initially for 99 years, with a potential 40-year extension. The agreement also entails a payment of £3.4 billion ($4.5 billion) from London to Port Louis, divided over the next few years. Starting in the late 1960s, the native Chagossians were forcibly displaced to secure military sites in the British Indian Ocean Territory. Legal battles, starting in the early 1970s, are still ongoing in British courts.

From the perspective of the British and American military establishments, the base on Diego Garcia is crucial. Given the chilly winds of naval rivalry in the Indian Ocean and China’s expansion in those waters, losing access to the territory could have long-term consequences. At the same time, the question of Mauritian sovereignty and territorial integrity is urgent, as is dealing with the claims of the exiled Chagossians. As Bashfield hints, these two objectives do not diverge. Yet, the conundrum persists. Such is the knot of Diego Garcia in the heart of the Indian Ocean.