THE recent decision by the Hague-based International Court of Justice that the Chagos Islands — with their huge US military base at Diego Garcia — are being illegally occupied by Britain has the potential to upend the strategic plans of a dozen regional capitals, ranging from Beijing to Riyadh.

For a tiny speck of land measuring only 38 miles in length, Diego Garcia casts a long shadow.

Sometimes called Washington’s “unsinkable aircraft carrier,” planes and warships based on the island played an essential role in the first and second Gulf wars, the invasion of Afghanistan and the war in Libya.

Its strategic location between Africa and Indonesia and 1,000 miles south of India, gives the US access to the Middle East, central and south Asia and the vast Indian Ocean. No oil tanker, no warship, no aircraft can move without its knowledge.

Most US citizens have never heard of Diego Garcia for a good reason: no journalist has been allowed there for more than 30 years and the Pentagon keeps the base wrapped in a cocoon of national security.

Indeed, Britain leased the base to the US in 1966 without informing either the British Parliament or the US Congress.

The February 25 court decision has put a dent in all that by deciding that Britain violated United Nations Resolution 1514 prohibiting the division of colonies before independence.

Britain broke the Chagos Islands off from Mauritius, a former colony on the south-east coast of Africa that Britain decolonised in 1968.

At the time, Mauritius objected, reluctantly agreeing only after Britain threatened to withdraw its offer of independence.

The court ruled 13-1 that Britain had engaged in a “wrongful act” and must decolonise the Chagos Islands “as rapidly as possible.”

While the ruling is only “advisory,” it comes at a time when the US and its allies are confronting or sanctioning countries for supposedly illegal occupations — Russia in the Crimea and China in the South China Sea.

The suit was brought by Mauritius and some of the 1,500 Chagos Islanders who were forcibly removed from the archipelago in 1973.

The US, calling it “sanitising” the islands, moved the Chagossians more than 1,000 miles to Mauritius and the Seychelles, where they have languished in poverty ever since.

Diego Garcia is the lynchpin for US strategy in the region. With its enormous runways, it can handle B-52, B-1 and B-2 bombers and huge C-5M, C-17 and C-130 military cargo planes.

The lagoon has been transformed into a naval harbour that can handle an aircraft carrier. The US has built a city — replete with fast-food outlets, bars, golf courses and bowling alleys — that hosts some 3,000 to 5,000 military personnel and civilian contractors.

What you can’t find are any native Chagossians.

The Indian Ocean has become a major theatre of competition between India, the US and Japan on one side, and the growing presence of China on the other.

Tensions have flared between India and China over the Maldives and Sri Lanka, specifically China’s efforts to use ports on those island nations.

India recently joined with Japan and the US in a war game — Malabar 18 — that modelled shutting down the strategic Malacca Straits between Sumatra and Malaysia, through which some 80 per cent of China’s energy supplies pass each year.

A portion of the exercise involved anti-submarine warfare aimed at detecting Chinese submarines moving from the South China Sea into the Indian Ocean.

To Beijing, those submarines are essential for protecting the ring of Chinese-friendly ports that run from southern China to Port Sudan on the east coast of Africa.

Much of China’s oil and gas supplies are vulnerable, because they transit the narrow Mandeb Strait that guards the entrance to the Red Sea and the Strait of Hormuz that oversees access to the oil-rich Persian Gulf. The US 5th Fleet controls both straits.

Tensions in the region have increased since the Trump administration shifted the focus of US national security from terrorism to “major power competition” — that is, China and Russia.

The US accuses China of muscling its way into the Indian Ocean by taking over ports, like Hambantota in Sri Lanka and Gwadar in Pakistan that are capable of hosting Chinese warships.

India, which has its own issues with China dating back to their 1962 border war, is ramping up its anti-submarine forces and building up its deep-water navy.

New Delhi also recently added a long-range Agni-V missile that is designed to strike deep into China, and the right-wing government of Narendra Modi is increasingly chummy with the US military.

The US even changed its regional military organisation from “Pacific Command” to “Indo-Pacific Command” in deference to New Delhi.

The term for these Chinese-friendly ports — “string of pearls” — was coined by Pentagon contractor Booz Allen Hamilton and, as such, should be taken with a grain of salt.

China is indeed trying to secure its energy supplies and also sees the ports as part of its worldwide Road and Belt Initiative trade strategy.

But assuming the “pearls” have a military role, akin to 19th-century colonial coaling stations, is a stretch. Most of the ports would be indefensible if a war broke out.

Diego Garcia is central to the US’s war in Somalia, its air attacks in Iraq and Syria, and its control of the Persian Gulf, and would be essential in any conflict with Iran.

If the current hostility by Saudi Arabia, Israel and the US toward Iran actually translates into war, the island will quite literally be an unsinkable aircraft carrier.

Given the strategic centrality of Diego Garcia, it is hard to imagine the US giving it up, or, rather, the British withdrawing their agreement with Washington and decolonising the Chagos Islands. In 2016, London extended the US lease for 20 years.

Mauritius wants the Chagos back, but at this point doesn’t object to the base. It certainly wants a bigger rent cheque and the right eventually to take the island group back.

It also wants more control over what goes on at Diego Garcia. For instance, the British government admitted that the US was using the island to transit “extraordinary renditions,” people seized during the Afghan and Iraq wars between 2002 and 2003, many of whom were tortured. Torture is a violation of international law.

As for the Chagossians, they want to go back.

Diego Garcia is immensely important for US military and intelligence operations in the region, but it is just one of some 800 US military bases on every continent except Antarctica.

Those bases form a worldwide network that allows the US military to deploy advisers and special forces in some 177 countries across the globe. Those forces create tensions that can turn dangerous at a moment’s notice.

For instance there are currently US military personnel in virtually every country surrounding Russia: Norway, Poland, Hungary, Kosovo, Romania, Turkey, Latvia, Lithuania, Estonia, Georgia, Ukraine and Bulgaria. Added to that is the Mediterranean’s 6th Fleet, which regularly sends warships into the Black Sea.

Much the same can be said for China. US military forces are deployed in South Korea, Japan and Australia, plus numerous islands in the Pacific. The US 7th fleet, based in Hawaii and Yokohama, is the navy’s largest.

In late March, US navy and coastguard ships transited the Taiwan Straits, which, while international waters, the Chinese consider an unnecessary provocation. British ships have also sailed close to Chinese-occupied reefs and islands in the South China Sea.

The fight to decolonise the Chagos Islands will now move to the UN general assembly. In the end, Britain may ignore the general assembly and the court, but it will be hard pressed to make a credible case for doing so.

How Britain can argue for international law in the Crimea and South China Sea, while ignoring the International Court of Justice on the Chagos, will require some fancy footwork.

In the meantime, Mauritius Prime Minister Pravard Jugnauth calls the court decision “historic,” and one that will eventually allow the 6,000 native Chagossians and their descendants “to return home.”

Conn M Hallinan blogs at dispatchesfromtheedgeblog.wordpress.com.