IT is a tale of inhumanity, cynicism and eye-watering official skulduggery. Implicated are a British prime minister, his cabinet colleagues, an American regime concerned only with its own global war games, and a slew of officials in the Foreign and Colonial Offices in Whitehall, which conspired to suppress the truth, subvert international law, and destroy the lives of more than 1500 people.

The year is 1965. The setting is 2000 miles east of Mombasa in Kenya and 1000 miles south-west of the southern tip of India. We’re in the Chagos archipelago in the Indian ocean. The largest island? You may have heard of it. It is called Diego Garcia.

The International Court of Justice in the Hague held this week that the United Kingdom unlawfully dismembered the territory from Mauritius and Britain is now “under an obligation to bring to an end its administration of the archipelago as rapidly as possible”. To understand the shameful train of events which led to this judicial condemnation of one of Britain’s last acts of unvarnished colonialism, we have to return to the beginning of this appalling story. Perfidious Albion barely covers what happened.

In 1965, colonialism was in full retreat across the globe. New states were approaching and embracing their independence. But in Whitehall? On the November 8, 1965, the United Kingdom created a new colony. They called it the British Indian Ocean Territory. Unbeknown to the Chagossians, their territories had been detached from Mauritius by Britain and folded into this new entity.

But why? At this point in our story, along comes a spider. The writing was on the wall for British rule in Mauritius and the Seychelles. The United States was keen to secure access to a permanent Indian Ocean port. It was less keen on dealing with the uncertainties of striking defence accords with new regimes in these emerging, post-colonial island states.

So here was the plan. Persuade the Brits to dismember a convenient island from one of their colonies before independence was declared. Leave it to them to do the dirty work of disposing of any inconvenient civilian populations – and the Americans could lease out a pristine site for its military manoeuvres in the region. The historical ironies of this are beyond satire, from the 1776 Declaration of Independence, to cajoling Harold Wilson to exercise the powers of King George III. Article 73 of the UN Charter is uncompromising. Colonial powers responsible for “territories whose peoples have not yet attained a full measure of self-government” must recognise “the principle that the interests of the inhabitants of these territories are paramount” and accept as a “sacred trust” the obligation to promote the well-being of the people of these territories to “the utmost”.

As the Chagossians soon discovered, the United Kingdom would grossly betray that trust. The legal basis for excising the archipelago from Mauritius was an exercise of her majesty’s royal prerogative. This was done secretively, without any parliamentary oversight or democratic process. The Order in Council which created the British Indian Ocean Territory empowered the colonial governor to make laws “for the peace, order and good government” of the territory and its people.

At London’s direction, he signally failed to do so. Instead, Sir Bruce Greatbatch used his plenary powers to expel the entire Chagossian population from their homes. The official paper trail is damning, smoothly cynical and casually racist. British functionaries painted a thin veneer of legality over their actions, but as the records make clear, many of the key actors involved in formulating the Whitehall line didn’t believe their own propaganda.

The argument was this. The Chagossians, Britain claimed, were temporary workers serving impermanent employment contracts. They were really Mauritians and Seychellois. One official compared them to “Spaniards who go daily to work in Gibraltar rather than to the permanent inhabitants of either Gibraltar or the Falklands Islands”. This was a lie. It was known to be a lie. They lied anyway.

Internal memos, turned up in subsequent litigation, make for bleak reading. Civil servants were looking for pretexts to “maintain the argument” that nobody lived on Diego Garcia permanently. “It is of particular importance that the decision taken by the Colonial Office should be that there are no permanent inhabitants,” one said. But that decision wasn’t the Colonial Office’s to take. The facts were incontrovertible.

As one Foreign Office lawyer observed in 1966, as the general expulsion of the population was contemplated, what was happening was “to certify, more or less fraudulently” that the permanent community of Chagossian islanders were really transient Mauritian workers. This wasn’t true, “however convenient we or the US might find it from the viewpoint of defence”. It is “important to avoid giving the impression that we are trying to get rid of these people,” another mandarin noted nervously. Another recommended “a policy of quiet disregard” for international law.

The permanent under-secretary at the Foreign Office encouraged his colleagues to be “very tough about this”. “The object of the exercise,” his directive read, “was to get some rocks which will remain ours. There will be no indigenous populations except seagulls. Unfortunately, along with the birds go some few Tarzans or Men Fridays whose origins are obscure, and who are being hopefully wished on to Mauritius etc.” The inhumanity and conceit of these remarks speaks for itself.

Some people were evacuated. Others were allowed to leave the islands to visit relatives or for medical treatment but were prevented from boarding the steamer back to their homes. As Judge Cançado Trindade summarised this week: “The Chagossians expelled from their homeland were abandoned in other islands in extreme poverty, in slums and empty prisons, in chronic poverty.”

With a military base to offer up to American cousins, for years Britain washed its hands of the scattered Chagossian people. Belated compensation cannot answer for that. The UK has continued to prevent the population from returning in all the years since.

Phillipe Sands QC – for the Mauritians – made his closing submissions in the case a few weeks after Boris Johnson resigned from the Cabinet. You may remember, Johnson claimed Britain was “headed for the status of a colony” to the EU if Theresa May’s deal with Brussels was agreed.

“The irony of his words will not be lost on those in this Great Hall,” Sands told judges. “The United Kingdom does not wish to be a colony, yet it stands before this court to defend a status as coloniser of others, of Mauritius, a significant part of whose territory it still administers. Unlike Mauritius, the coming “colony” of the former British foreign secretary’s imagination is not in danger of having its people forcibly removed and then prevented from returning.”

The United Kingdom now “fully accepts that it treated the Chagossians very badly at and around the time of their removal and it deeply regrets that fact”. That regret rings terribly hollow, and the tears crocodile, coming from a government of swaggering John Bulls more inclined to give self-righteous lectures about the bright side of empire than recognise the tainted history which is a great part of Britain’s national inheritance.

What happened – what the UK did – to the Chagossians deserves to be far more widely known and understood than it is, despite the tireless efforts of champions such as Olivier Bancoult to hold this country to account for what it has done and continues to do.

It recalls the sharp old line. You know why the sun never set on the British empire? Because even God couldn’t trust the British in the dark. But in the Peace Palace in the Hague this week, at least, a little justice, a little truth, at last – some accountability.