The Chagos Treaty – And the New Colonialism

This article was re-posted with the permission of Dr Dan Boucher.

The Diego Garcia Military Base and British Indian Ocean Territory Bill[1] had its First Reading in the House of Commons on 15 July, shortly before recess. Every substantive scrutiny stage through which the Bill must travel consequently lies ahead of us. The entire substance of, first, House of Commons and then House of Lords scrutiny of the Bill cannot begin until September at the earliest. The Commons Second Reading debate is scheduled for today, September 9th.

The Explanatory Notes accompanying the Bill state: ‘The Treaty will enter into force only after both Parties to it have completed their domestic procedures for its implementation and notified each other that they have done so. This Bill forms part of the UK’s implementation of the Treaty, and also makes provision which results from the Treaty, including in relation to British nationality. The majority of provisions within the Treaty do not require legislation to implement them and therefore are not reflected in the Bill.’

This means that the treaty cannot enter into force until after the Diego Garcia Military Base and British Indian Ocean Territory Bill has been passed so it certainly is not a done deal. In this context there is an opportunity to subject both the Treaty and the Bill to much closer scrutiny and doing so will raise huge questions. A number of different concerns have come into focus over the summer including questions about the cost,[2] the implications for defence raised by the US Congress Appropriations Committee[3] and the nature of the response of the UK Government to a threatened legal challenge.[4] No doubt these will play a significant role in the debate on the Bill. There are, however, an additional set of concern from the perspective of decolonisation and self-determination which it is the purpose of this long-read article explore.

The treaty between the UK and Mauritius concerning the Chagos Archipelago including Diego Garcia CP 1334, signed on 22 May 2025, contains the following statement which underpins the entire rationale for the UK transferring the Chagos Archipelago to Mauritius: ‘Recalling the unique circumstances and history of the Chagos Archipelago, and mindful of the need to complete the process of decolonisation of Mauritius;’[5] (Emphasis added)

The basic justification for the treaty is that only part of Mauritius was decolonised in 1965, that this was unjust, and decolonisation should now be completed. It is the purpose of this article to argue that, far from assisting in this process of decolonisation, the effect of both the treaty and the bill are to compound rather than alleviate the presenting injustices because they are based on a misunderstanding of decolonisation. In what follows this analysis seeks to rise to this challenge, first, by setting out the arguments that the treaty completes decolonisation, and then explaining why it does not, highlighting the changes that would be required in order to secure decolonisation.

Background: The Facts of the Case
Before turning to these arguments, however, it is important to set out the basic facts of the case. Prior to the decolonisation of Mauritius, the USA expressed a desire to use part of the colony as a naval base. The United Kingdom wanted to both help its main ally and decolonise and proposed a solution based on the fact that in addition to the island of Mauritius the colony was unusual in that it also included the Chagos islands, located 1,339 miles away, and with a small population said to range from over 1000 to 2000 people.[6] The UK proposed that prior to decolonisation of Mauritius, the Chagos islands be separated to form a small new colony called the British Indian Overseas Territory (BIOT) and that the island of Diego Garcia in the Chagos archipelago should be used by the United States as a military base.

The Mauritius Council of Ministers agreed to this proposition in 1965, and BIOT was formed on 8 November 1965. This meant that when, in 1968, Mauritius gained independence, the Chagos islands continued to be a UK colony. In a deeply disturbing and shameful act, the residents of BIOT were then forcibly removed from the islands between 1968 and 1973. All the residents of Diego Garcia were removed by the first part of 1971 and the island was then handed over to the United States in the July. Today it is understood to constitute a large American naval and air base with a much smaller UK military presence.

The Argument that the Treaty Corrects an Historic Injustice
Although the Mauritius Council of Ministers accepted the proposal to detach the Chagos islands in 1965, the Republic of Mauritius decided to contest the arrangement in the 1980s. They claimed that in the same way the doctrine of ‘territorial integrity’ pertains to a sovereign state, so to it pertains in relation to a colony until such a time as full decolonisation has taken place and the people of the erstwhile colony are afforded the right of full self-determination. On this basis the UK should not have sought to separate the Chagos Islands from the rest of the colony before decolonisation. In a non-binding judgement in 2019 the International Criminal Court largely agreed with this argument, and stated that the division of the colony of Mauritius before decolonisation was not legal because it did not respect the territorial integrity of the pre 8-November 1965 colony:

‘The Court recalls that the right to self-determination of the people concerned is defined by reference to the entirety of a non-self-governing territory. Both State practice and opinio juris at the relevant time confirm the customary law character of the right to territorial integrity of a non-self-governing territory as a corollary of the right to self-determination. No example has been brought to the attention of the Court in which, following the adoption of resolution 1514 (XV), the General Assembly or any other organ of the United Nations has considered as lawful the detachment by the administering Power of part of a non-self-governing territory, for the purpose of maintaining it under its colonial rule. States have consistently emphasized that respect for the territorial integrity of a non-self-governing territory is a key element of the exercise of the right to self-determination under international law. The Court considers that the peoples of non-self-governing territories are entitled to exercise their right to self-determination in relation to their territory as a whole, the integrity of which must be respected by the administering Power. It follows that any detachment by the administering Power of part of a non-self-governing territory, unless based on the freely expressed and genuine will of the people of the territory concerned, is contrary to the right to self-determination.’

In this context, although the Council of Ministers of Mauritius had decided to acquiesce to the British request that the colony be divided, the Judgement states:

‘The Court observes that when the Council of Ministers agreed in principle to the detachment from Mauritius of the Chagos Archipelago, Mauritius was, as a colony, under the authority of the United Kingdom. Having reviewed the circumstances in which the Council of Ministers of the colony of Mauritius agreed in principle to the detachment of the Chagos Archipelago on the basis of the Lancaster House agreement, the Court considers that this detachment was not based on the free and genuine expression of the will of the people concerned.’

The Judgement went on:

‘The Court concludes that, as a result of the Chagos Archipelago’s unlawful detachment and its incorporation into a new colony, known as the BIOT, the process of decolonization of Mauritius was not lawfully completed when Mauritius acceded to independence in 1968.’

In light of the Judgement, the UK Labour Government argues that the only way forward is for the UK to terminate BIOT and transfer the Chagos Islands to Mauritius and thereby complete the decolonisation of the pre 8th November 1965 colony. This does not mean that the US base on Diego Garcia necessarily need close so long as the Republic of Mauritius agrees to its remaining, subject to their sovereignty, rather than UK sovereignty.

Both the treaty signed on 22 May, and the Diego Garcia Military Base and British Indian Ocean Territory Bill, rise to this challenge. First, they make provision for the transfer of the Chagos islands to Mauritius, establishing Mauritian sovereignty over the islands. Second, they make provision for the US base to remain on Diego Garcia subject to: i) the island now being under Mauritian sovereignty and ii) the UK paying for the right to lease Diego Garcia for one hundred years and then making it available to the USA.

The Argument that the Treaty Compounds the Existing Injustice
Many commentators have been quick to point out that the above approach is problematic in the sense that it treats the 2019 Judgement as binding when it is no such thing. While this is a point well made, we should not allow it to obscure the more important point that even if the Judgement was binding the argument that the treaty and Diego Garcia Military Base and British Indian Ocean Territory Bill constitutes the way forward, would still suffer from at least two fatal flaws:

Decolonisation: Self-Determination and Territorial Integrity?
In the first instance, decolonisation is a process that relates not just to a territory but a territory and its people. This point is made crystal clear by the UN definition of decolonisation, provided by its Declaration on Decolonisation, UN Resolution 1514. The completion of decolonisation spoken of by the treaty must be consistent with this, the seminal definition of decolonisation, which is about people and depends on their self-determination. The first two Articles state:

1. The subjection of peoples to alien subjugation, domination and exploitation constitutes a denial of fundamental human rights, is contrary to the Charter of the United Nations and is an impediment to the promotion of world peace and co-operation.

2. All peoples have the right to self-determination; by virtue of that right they freely determine their political status and freely pursue their economic, social and cultural development.[7]

While the people living on the island of Mauritius were not removed, with the result that the part of the former colony that was decolonised in 1968 could be decolonised without engaging with any actors beyond what became the Republic of Mauritius, the same cannot be said of the British Indian Ocean Territory in 2025 because the people of the Chagos Islands were removed by the colonial power between 1968 and 1973.

How should we then proceed?

Territorial Integrity Without Self-Determination
The treaty works on the basis that with the Chagossians removed from the islands, we should simply seek to restore things by reference to the principle that is left, territorial integrity. In the absence of the Chagossians, so the argument goes, the imperative is to restore the Chagos Islands to Mauritius. It is bad enough that the people of the Chagos islands were removed. To then effectively argue that because of this the islands can no longer be decolonised because the people of the Chagos islands have been subject to the grave injustice of forced removal would be perverse in the extreme. The Chagos islands, whether peopled or not, are part of the territorial whole that was wrongly cut in two on 8 November 1965 and the presenting moral imperative, is now their restoration. Of course, those expressing this perspective are quick to point out that it does not mean that the welfare of the Chagossians living away from their islands is now irrelevant. To adopt that position would be equally perverse which is why the treaty and bill allocates £40 million for the purpose of supporting them.

Territorial Integrity With Self-Determination

However, this argument does not stand up to scrutiny. While it rightly leaps on the point that it would be quite wrong to say that BIOT cannot be decolonised because its people have been removed, it is equally wrong and perverse to say that self-determination is not possible because the people of the Chagos islands have been removed from the islands. In a context where the actual definition of decolonisation has self-determination at its heart, the above position is unsustainable. Treaties are an instrument of the international community which embraces the people of the Chagos Islands, the Chagossians. They live principally in three sovereign states all of which are members of the UN: the UK, Mauritius and the Seychelles. Moreover, we must also appreciate that while territorial integrity is important, it is secondary to self-determination which defines its boundaries which can be changed as a result of self-determination’. The truth is that territorial integrity apart from self-determination confounds rather than gives expression to decolonisation.

In coming to terms with the above it is vital to keep in view the key aspects of international law developed for the purpose of establishing whether decolonisation has taken place and its impact of territorial integrity. These exist in a series of UN Resolutions 567, 648, 742 and 1514 defining: the ‘Factors indicative of the free association of a territory on equal basis with the metropolitan or other country as an integral part of that country or in any other form.’ Crucially they recognise that while detachment should not take place under pressure from the colonial power, neither should the colonial territorial integrity be regarded as absolute. Self-determination can result in a former colony accepting its colonial territorial integrity as its self-determined territorial integrity as a sovereign state, but it can equally be changed through self-determination. The critical point here is not to treat the colonial ‘territorial integrity’ as forever frozen in time, concreting in the territorial determinations of the colonial power which may have been quite arbitrary, but rather ensuring that decolonisation constitutes genuine self-determination by the peoples of the territory in question, which might result in: i) the affirmation of the colonial territorial integrity, either as a sovereign state or in a determination to continue to be governed in part by the colonial power or ii) the rejection of the colonial territorial integrity.[8] This point is well established within the life of: a) largely self-governing colonies that self-determine to remain governed by the colonial power in some respects, b) former colonies and c) existing sovereign states. Self-determination, for example, resulted in the break-up of the colony of India after decolonisation into three separate sovereign states: India, Pakistan and Bangladesh. It also resulted in the break-up of the sovereign state of Czechoslovakia into two new sovereign states the Czech Republic and Slovakia.

Although the non-binding judgement of 2019 is used to justify the May 22nd treaty that adheres to the borders of the pre-November 8th 1965 colony, the truth is that the judgement actually makes the point that the colonial territorial integrity cannot be regarded as frozen in time. The Judges stated: ‘any detachment by the administering Power of part of a non-self-governing territory, unless based on the freely expressed and genuine will of the people of the territory concerned, is contrary to the right to self-determination.’ This actually goes even further in emphasizing the importance of self-determination over territorial integrity in that it clearly implies that changing the territorial integrity of a colony is even acceptable prior to decolonisation, if this determination is ‘based on the freely expressed and genuine will of the people of the territory concerned.’ On that basis, had the UK not sought the break-up of the pre-November 8 1965 colony, the option would have been open to the Chagossians to indicate a desire to form their own polity before decolonisation (as well as after) which, if validated through an appropriate mechanism, a referendum, could have resulted in their becoming entirely separate on decolonisation.[9] Furthermore, and crucially for our purposes, we must recognise the implications of two factors that clearly make the likelihood of a Chagossian self-determination for a new territorial integrity much more likely than would be the case in relation to most attempted expressions of decolonisation.

In the first instance, the 1965 boundaries of the colony were highly unusual because the distance between Mauritius and the Chagos islands is 1,339 miles. To put that in some kind of context, that is the same distance between Kent, in the South of England, to North Africa. In the second instance, and not surprisingly the peoples of Mauritius, on the one hand, and the Chagos islands, on the other, are different both on the bases of ethnicity and religion.

The truth is that the inclusion of these different places in the same colony would appear to be born of a certain arbitrariness arising from regard for the convenience of the imperial power at the time rather than regard for the sense of ‘belonging’ informing the boundaries of the communities in question. While, of course, we cannot say what would have happened had the whole pre-November 8th 1965 colony decolonised in 1968, and embraced self-determination, we can say that if ever there was a colony whose self-determination might result in changes in its territorial integrity, this was it.[10] In this context it would be particularly wrong to assume that the Chagos islands should simply be ‘returned’ to Mauritius because of the importance of upholding a colonial territorial integrity, frozen in time by means of the removal of the people of the Chagos islands.

‘A free and genuine expression of the will of the people concerned’?
In the second instance, the only possible response to the first fatal flaw, set out above, quickly reveals itself to be similarly flawed. Specifically, the point might be made that the above difficulty can be dealt with by means of arguing that that the Republic of Mauritius can self-determine on behalf of the Chagossians because a significant number of Chagossians live on the island of Mauritius and are Mauritian citizens. This argument, however, does not stand up to scrutiny.

In the first instance, most Chagossians do not live in Mauritius. If we want to complete decolonisation of the pre-8 November 1965 colony, we have to offer the people of the part of that colony that was not decolonised, the Chagos Islands, self-determination. This is not provided by a mechanism that does not involve at least a majority of Chagossians living in the world today.

In the second instance, the Chagossians who live in Mauritius do so because they were forcibly removed from the Chagos islands to Mauritius against their will. The Mauritians received £3 million in compensation for agreeing to this and were then implicated in the forced removal of the Chagossians to Mauritius by means of agreeing to accept them against their will. In this context it is no surprise that research suggests that Chagossians living on Mauritius are, on average, more likely to be significantly poorer than other citizens. Moreover, there is evidence of discrimination by Mauritians against Chagossians living in Mauritius and, while some Chagossians may say that the Republic of Mauritius represents them, many Chagossians have made it very clear that the Republic of Mauritius in no way speaks for them and a growing number of Chagossians are leaving Mauritius.[11]

In this context, if it was problematic to elicit credible consent from a decision made by the Mauritian Council of Ministers in 1965 because, notwithstanding the dignity of a significant measure of self-government expressed through the existence of that Council of Ministers, Mauritius was at the time a UK colony, how much more difficult to infer the consent of the Chagossians in the May 22nd treaty. Indeed, the principle that was contravened, and resulted in the 2019 non-binding judgement, is contravened to an even greater extent by the May 2025 treaty and the Diego Garcia Military Base and British Indian Ocean Territory Bill. As noted above, the point about most forms of colonial government is not that they are without elements of self-government, as demonstrated by the fact that when Mauritius was a colony in 1965 it had a Council of Ministers, which was asked by the UK to make a decision. If this was not valid because Mauritius at the time did not enjoy full self-government in the sense that some decisions were made by the UK government that was ultimately sovereign, how much greater the offence caused by implying Chagossian consent for the treaty on account of the presence of some Chagossians in Mauritius when: i) most Chagossians don’t live in Mauritius, ii) those that do were forcibly moved to Mauritius against their will with the co-operation of the Republic of Mauritius and iii) unlike the Mauritian Council of Ministers in 1965, there was no Chagossian Council of Ministers in 2025 and the Chagossians living on Mauritius were not even asked for a decision.

When viewed in the context of a full understanding of the principles of decolonisation as they relate both to ‘territorial integrity’ and ‘self-determination’, we have to say of the 22nd May treaty that there can have seldom been a better example of a project hoisted on its own petard. If the splitting of the colony in 1965 was illegitimate because it was contrary to self-determination on account of not permitting the colony to express a sufficiently unencumbered decision, then so is implying Chagossian consent for the Chagos islands rejoining Mauritius in 2025 without affording them any opportunity to make that decision.

The Way Forward
In a context where it is not possible to decolonise any portion of the globe without the people that pertain to that portion of the globe, in our case the Chagossians, and where it is not possible to argue that they are afforded self-determination on account of the fact that a minority of them live in Mauritius, this does not create a situation where we are justified in giving up on decolonisation. In this we are assisted in two ways: First, there are only around 10,000 Chagossians living in the world today. Second, for the most part they only live in three countries, all of which are members of the United Nations: the Seychelles, Mauritius and the United Kingdom. In a context where we have just witnessed the development of a scheme enabling 5.7 million people from 27 states apply for and gain UK residency through the EU Settlement Scheme, and votes in the UK, the task of locating 10,000 Chagossians to give them a vote in a self-determination referendum is not beyond our reach.[12] They should be given the choice of self-determining to be part of Mauritius or self-determining to be a largely self-governing British Overseas Territory that could in future potentially become fully independent. (The infrastructure does not exist on the island at present to offer immediate independence, not least because the Chagossians have been removed).

In response to this some might object that it would be too expensive to enable the Chagossians to return to the Chagos islands. Two points must be made in response to this. In the first instance, given the centrality of self-determination to decolonisation, this objection would amount to another way of saying we are not committed to decolonisation, negating the stated purpose of the May 22 treaty. In the second instance, the argument deployed by the UK Government that it would be too expensive to facilitate the return of the Chagossians to their home is now very difficult to sustain, in the context of: i) claiming a commitment to decolonisation, and ii) the vast amount of money that the UK Government is proposing to pay the Republic of Mauritius annually so that the UK can continue to invite the United States to maintain its military base on Deigo Garcia.[13] Rather than correcting the injustice of the forced removal of the Chagossians, this greatly compounds the offense by demonstrating that what the UK government told them in 2016 was not true. A huge amount of money could have been found to correct a huge wrong but the UK Government, despite its protestations of apology, was not prepared to pay the Chagossians what it now transpires it is prepared to pay the Mauritians. Indeed, care has been taken to compound the sense of injustice to the greatest possible extent, because the patronising rather than empowering sum afforded the Chagossians amounts to just 40% of what the Mauritians will get in just the first year and which they will then continue to get for another 99 years while the Chagossians receive nothing. It is hard to conceive of an arrangement better designed to inflame the original injustice.

Conclusion
In conclusion, the notion that the 22 May treaty and the Diego Garcia Military Base and British Indian Ocean Territory Bill constitute an appropriate means of completing decolonisation does not stand up to scrutiny. Rather than making things better, they make things much worse. If the UK Government is seriously committed to completing decolonisation, then it must take the necessary steps to facilitate Chagossian self-determination. This would involve inviting Chagossians from the Seychelles, Mauritius and the UK to register to vote (subject to establishing their claim to be Chagossian) in a self-determination referendum with the options of the Chagos Islands: i) being transferred to Mauritius or ii) becoming a self-governing British Overseas Territory that might in the future become an independent state. No one is suggesting that this would be easy or straight-forward but it would be possible. For so long as this is not done, the lie that the treaty of 22 May is about completing decolonisation must be called out. If the UK Government is not prepared to do the right thing, then NGOs should embarrass them into doing so by providing the process themselves, unofficially, to demonstrate that it is possible, and to thereby pull from the treaty of 22 May, and the Diego Garcia Military Base and British Indian Ocean Territory Bill, any pretence of moral validity.

In the history of colonisation, the experience of the Chagossians really stands out. They were not merely subject to colonial rule that prevented self-government. They were subject to a much more profound disinheritance that took from them both self-government and their homeland. If there is a moral imperative today to decolonise those subject to a colonisation that took from them self-government but not their homeland, how much greater the moral imperative to decolonise those subject to a colonisation that took from them both their self-government and their homeland. And how much greater the moral difficulty caused by a treaty pretending to complete decolonisation when instead its provisions lay the foundations for an even greater sense of injustice because of the transfer of huge sums of money to another people, the Mauritians, to provide payment for the use of the lands of the Chagossians that could be used to secure their decolonisation.

Dr Dan Boucher

September 2025

[1] https://bills.parliament.uk/bills/4004

[2] https://www.telegraph.co.uk/politics/2025/08/10/revealed-chagos-deal-to-cost-10-times-what-starmer-claimed/

[3] https://order-order.com/2025/08/05/exc-trump-could-still-block-starmers-chagos-deal-after-house-appropriations-intervention/

[4] https://conservativepost.co.uk/government-accused-of-intimidation-in-chagos-islands-legal-battle/

[5]https://assets.publishing.service.gov.uk/media/682f25afc054883884bff42a/CS_Mauritius_1.2025_Agreement_Chagos_Diego_Garcia.pdf

[6] Who were the people living in the Chagos Archipelago before 1965? and Chagossians – Wikipedia

[7] https://www.ohchr.org/en/instruments-mechanisms/instruments/declaration-granting-independence-colonial-countries-and-peoples

[8] 567 (VI). Future procedure for the continuation of the study of factors which should be taken into account in deciding whether a territory is or is not a territory whose people have not yet attained a full measure of self-government https://docs.un.org/en/A/RES/567(VI)

648 (VII). Factors which should be taken Into account in deciding whether a Territory is or is not a Territory whose people have not yet attained a full measure of self-government https://docs.un.org/en/A/RES/648(VII)

742 (VIII). Factors which should be taken into account in deciding whether a Territory is or is not a Territory whose people have not yet attained a full measure of self-government https://docs.un.org/en/A/RES/742(VIII)

Principles which should guide Members in determining whether or not an obligation exists to transmit the information called for under Article 73e of the Charter.

https://digitallibrary.un.org/record/206178?ln=en&v=pdf

[9] In this context, when the qualification of the judgement is read in tandem with the ‘Factors indicative ..’ resolutions, see footnote 8, it is clear that the Chagossians could have self-determined to remain associated with the UK in the same way as Gibraltar, while the rest of the pre November 8th 1965 colony self-determined to become an independent sovereign state.

[10] On this point it is interesting that ethnic people group difference and geographical distance creates, for some purposes, a presumption in favour of the idea that the territory in question should have its own government: see, ‘the Principles which should guide Members in determining whether or not an obligation exists to transmit the information called for under Article 73 e of the Charter’, as part of the UN Declaration on Decolonisation, which obtains in relation to a colonial power when a territory is deemed to be a colony state: ‘IV Prima facie there is an obligation to transmit information in respect of a territory which is geographically separate and is distinct ethnically and/or culturally from the country administering it.’ See UN Resolution 1541 (XV).

[11] https://www.express.co.uk/news/politics/2004871/david-lammy-chagossians-mauritius

‘One main finding of the report [Human Rights Watch Report] was that many of the Chagossians who were forcibly displaced to Mauritius by the UK government to allow the US to build a military base on their island suffered, and still suffer, poverty, stigma, and discrimination a half century later.’

https://www.hrw.org/news/2023/05/08/ensure-chagossians-have-access-mauritius-prosperity-too

https://en.wikipedia.org/wiki/Expulsion_of_the_Chagossians#:~:text=The%20United%20Kingdom%2C%20at%20the%20request%20of%20the,are%20today%20known%20as%20Chagos%20Islanders%20or%20Chagossians.

[12] ‘As of 31 December 2024, there have been 8.4 million EUSS applications made since the scheme was launched on 28 August 2018. These applications relate to 6.3 million people (some people will have multiple applications). Of these, 5.7 million people currently hold a grant of status through the scheme (4.1 million settled status and 1.7 million pre-settled status).’ How many people have been granted settlement via the EU Settlement Scheme? – GOV.UK

[13] https://questions-statements.parliament.uk/written-statements/detail/2016-11-16/HLWS257