A remote and obscure tropical atoll in the Indian Ocean would seem an unlikely stage for the latest act of the United Kingdom’s unfolding Brexit drama and the newest challenge to U.S. global power projection. Yet in The Hague on February 25, 2019, the International Court of Justice (ICJ) ruled London’s control of the Chagos Islands “illegal” and that the isolated archipelago should be returned to Mauritius “as rapidly as possible.”

While nonbinding, the verdict calls into question the legitimacy of the UK’s administration and the major U.S. military presence on the islands’ largest atoll, Diego Garcia. Past analysis has described how the Chagos forms “one of the most important pieces of real estate” in the Department of Defense’s “strategic arsenal.”

Diego Garcia provides a vital base for U.S. air and maritime forces operating in the Indian Ocean or projecting power further afield. Its 12,000-ft runways are large enough to handle long-range bombers such as the B-1, B-2 or B-52, operating as far afield as North Africa, the Persian Gulf, Afghanistan or South China Seas. Described as an “unsinkable aircraft carrier,” Diego Garcia has previously been used to launch strike missions in both Gulf Wars and the invasion of Afghanistan. Its sheltered lagoon also provides an ideal base for prepositioned naval support vessels and offers a safe location for rearming and resupplying surface combatants and the submarine fleet.

Given a steady reduction in the U.S. military’s wider global footprint in recent decades, the naval support facility represents a vital node in the thin chain of U.S. bases between East Africa (Djibouti), the Persian Gulf, South-East Asia and Australia. The site is also home to a small British military contingent, including personnel from the Royal Navy and Marines, who also act as the islands’ civil administration.

The UK government has previously committed to return the islands “when no longer required for defence purposes.” However owing to these ongoing military activities, it has argued that its strategic location in the Indian Ocean means the archipelago plays too important a role “in regional and global security, helping to keep the UK, America and other allies, including Mauritius, safe” to give it up in the foreseeable future.

Responding to the ICJ’s recent verdict, the UK Foreign Office has noted that “this is an advisory opinion, not a judgement” but that “we will look at the detail of it carefully.” So might London’s control—and U.S. military presence on Diego Garcia—now be called into question?

Both have certainly faced controversy and opposition in the past. The UK has administered the small clutch of islands it calls the British Indian Ocean Territory since 1965 when London separated the Chagos from its colony Mauritius ahead of that nation’s independence in 1968. Recognizing the strategic importance of the isolated archipelago as a logistics hub and staging post, in 1966 the UK agreed to a fifty-year lease with the U.S. Department of Defense (later extended in 2016 out to 2036).

To make way for the U.S. military presence, London expelled the inhabitants of the islands between 1968 and 1973, with around 1,800 Chagossians forced to leave their homes. Many settled in Mauritius or in the UK, and have since undertaken legal proceedings against the UK government to seek compensation for “official wrongdoing” and a right of return to the islands.

The island nation of Mauritius has also claimed sovereignty, arguing that as a British colony it had been forced to give up jurisdiction over the Chagos Islands or else see its wider bid for independence blocked. The Mauritian authorities have continued to clash with London over maritime demarcations, fishing rights and London’s efforts to re-classify the archipelago as a marine reserve—a controversial move that would not affect the U.S. military presence, but would block the islands’ former inhabitants from returning to the area.

Despite this tumultuous history, the ICJ’s latest verdict represents a major victory for Mauritius and for Chagossian campaigners. While advisory and non-binding, it increases the diplomatic pressure on the UK government and its U.S. tenants to evacuate the Chagos Islands, or to at very least renegotiate their arrangements with Mauritius, which has the backing of both global and regional supporters. The ICJ ruled thirteen-to-one in Mauritius’s favour, with the majority including judges from Russia and China. The lone dissenting vote came from the sole U.S. representative on the bench.

The UK media has been quick to draw links between the ICJ ruling and the UK’s decision to leave the European Union—the Financial Times calling it a “moral blow to the UK on the world stage when Brexit will be leaving [the country] with a shaky diplomatic influence on institutions such as the UN General Assembly.” In 2017, the Assembly passed a resolution to refer the matter to the ICJ by ninety-four-to-fifteen. Among the sixty-five nations choosing to abstain were key European and other allies, including France, Germany and Canada, leaving the UK diplomatically isolated. Later that year, London suffered another political blow when it lost its place on the ICJ for the first time in the court’s seventy-one-year history, with rising power India able to secure more votes.

The UN General Assembly is now likely to look at the Chagos issue again, following the ICJ’s ruling, given its traditional lead on matters of decolonization. Given ongoing uncertainty about what a new “Global Britain” will look like in trading and foreign policy terms, this latest challenge to the UK’s administration of the Chagos Islands poses an early test for post–Brexit diplomacy.

The Foreign Secretary, Jeremy Hunt, has spoken of defining Britain’s post–Brexit role in the world as an “invisible chain” linking democracies and promoting values as much as interests. The UK’s Defence Secretary, Gavin Williamson, has meanwhile called for a more robust stance against actors such as Russia or China who are seen to undermine the rules-based world order. This includes dispatching the new HMS Queen Elizabeth aircraft carrier to the South China Seas, where Beijing has been building its own network of contested island bases. At the same time, the UK Ministry of Defence has indicated a desire to rebuild its presence “East of Suez,” including through investments in new bases in the Persian Gulf and South-East Asia, and maintenance of the permanent UK military presence on Diego Garcia.

Caught between these two impulses, the question of whether the UK is ready to defy the ICJ and UN General Assembly to keep a hold on Diego Garcia, and open itself up to accusations of hypocrisy, may prove a test case for this shifting foreign policy as London grapples with how it wants to define itself after Brexit. The leader of the Labour opposition, Jeremy Corbyn, was quick to welcome the ICJ ruling and called on Downing Street to “correct this injustice” against islanders “disgracefully forced from their homes… to make way for a U.S. military base.” The United States too continues to face a moral challenge to its soft power and influence. China can be expected to cite America’s defiance of the ICJ as legitimizing their own controversial base-building efforts in the South China Seas and elsewhere.

Assuming the United States is forced to deal with a new landlord after a possible UK withdrawal, Mauritius has been quick to emphasize it does not plan to disrupt the U.S. military presence. Whether or not this is simply a negotiating tactic or a signal of genuine intent, the Mauritian government is nonetheless likely to leverage basing rights for Diego Garcia in return for concessions from the United States.

Even if America were able to secure a new lease to remain, domestic political support in Mauritius for a continuing U.S. presence could not be assumed. Overseas U.S. bases in Japan, South Korea, Turkey and Europe have all seen vocal opposition, protests and other controversies in recent years. Japanese Prime Minister Shinzo Abe is currently facing down fierce local opposition to plans to relocate the U.S. base on Okinawa, following a referendum that saw over seventy percent of voters reject plans for more construction for the U.S. Marine Corps site.

The Chagos Islands, then, remain a vital strategic anchor for the United States and UK military in the Indian Ocean—uniquely positioned to support power projection within the region and further afield. At the same time, the ICJ’s recent ruling, the deep uncertainty around Brexit and the pressure to show robust moral leadership in defense of a rules-based world order all pose new challenges to both the U.S. tenant and UK landlord. Whether the remote and tropical lagoon of Diego Garcia proves safe harbor from the current political and diplomatic storms of Brexit may shape the wider geopolitics of the Indian Ocean for decades to come.

James Black is a senior analyst in the Defence, Security and Infrastructure (DSI) programme at RAND Europe, part of the global RAND Corporation. His research focuses on defense strategy and the impact of emerging geopolitical, societal and technological trends, including also the defense and security implications of the UK’s decision to leave the European Union.