President Donald Trump had some basis for his startling linkage of Greenland and the Chagos Islands, known unromantically in British officialdom as BIOT, the British Indian Ocean Territory. Both are Overseas Territories: one Danish, one British. Britain’s present approach to both has been based firmly on international law. Mr Trump’s has been based, as his adviser Steve Miller has put it, on “power”. Whether deliberately or accidentally, Mr Trump has added not only to Britain’s foreign policy problems but also potentially complicated domestic party politics.

Greenland’s legal status is clear enough. It is part of the Kingdom of Denmark, its status endorsed by a referendum of Greenlanders in 2008. Its international recognition goes back centuries, most recently acknowledged by a finding of the Permanent Court of International Justice in 1933, which sorted out a Danish-Norwegian dispute.

The Chagos Islands are more complicated. Britain’s decision to separate them from colonial Mauritius just before Mauritian independence in 1968 has long been controversial. It became tangled up with British accountability for the brutal deportation to the main part of Mauritius of plantation labourers and their descendants who had been brought to the islands to work there. They were removed because the point of keeping the islands was to continue hosting the large US base on the biggest island, Diego Garcia, and the Americans weren’t keen on having a local civilian population.

Mauritius has for years laid claim to the islands, not for any particular concern for its former small population but because they challenged the legality of the last-minute split of Mauritian territory. In 2019, its case was upheld by the International Court of Justice (the ICJ, successor to the court that confirmed Danish sovereignty in Greenland). That same year, the UN General Assembly voted 119-to-6 to support Mauritius’s claim.

These events put Britain in an awkward position. The same ICJ had upheld Ukraine’s rights in several cases against Russia; and the UN General Assembly had voted 141-to-5 to condemn Russia’s second invasion — almost the same numbers who had voted against the British position on the Chagos Islands. We were in the difficult position of, rightly, condemning Russia for failing to respect the findings of both organisations on Ukraine, while ignoring them ourselves in the Indian Ocean. It was hard to ignore possible double standards.

Complicating this more, we were also joining allied Indo-Pacific efforts to constrain China’s expansionism, particularly in the South China Sea. This often involves Chinese sovereignty claims on territorial control of reefs and atolls despite international law tribunals regularly finding that they don’t belong to them. We had only two territories in the region: Pitcairn (of which I was once non-resident governor, with a population of 45 in the middle of the Pacific) and the Chagos Islands, now with no permanent residents. The fact that our own claim to the latter had been found to be unlawful somewhat weakened our moral position with China.

To its credit, the Conservative government moved towards a position more consistent with the law. The easiest solution would have been to hand the islands to Mauritius. But the Government would have wanted to stay involved for American reasons. The value of the Diego Garcia base to the US is not widely realised. I was involved in the mid-Noughties in a review of what assets we brought to the US-UK relationship. The base on these islands that few people had heard of ranked in the top three items the Americans valued. To safeguard this — and to keep getting the credit as loyal allies — the Conservative and then Labour governments kept ourselves in the loop by securing a 99-year lease to the base from the new Mauritian owners, on the basis that we would sublet it to the US.

After some hesitation, the new Trump Administration enthusiastically endorsed this arrangement. In May last year, Secretary of State Rubio “commended both the United Kingdom and Mauritius for their leadership, vision and commitment to ensure that Diego Garcia remains fully operational for the duration of this agreement”. The agreement secured “the long-term, stable and effective operation” of the base. He added that “President Trump expressed his support for this monumental achievement during his meeting with Prime Minister Starmer at the White House”.

Why has Mr Trump’s reversal now affected our domestic politics? This is a time when to manage the extraordinarily rough international waters, the responsible British parties should work together as far as possible in the national interest, as clearly happened last week over Greenland. But Mr Trump’s comment on the Chagos Islands tapped on a weak point in this unity. The Labour government completed the deal with Mauritius that the Conservatives had started. Reform had opposed it, and Mr Farage very likely had a hand in the US administration’s earlier doubts about it. In opposition the Conservatives turned on their own deal, aligning with Reform. Andrew Rosindell, one of the few MPs with a genuine longstanding interest in overseas territories, was made a foreign affairs spokesman: he has long opposed any handovers of remaining bits of Empire to anyone, on principle. Last week he joined Reform.

I wrote a week ago that the Conservatives might be tempted by Mr Trump’s online criticism of the Chagos agreement to peel off from the new consensus of standing firm in dealing with the Trump regime. I hoped that they might at least stay quiet on this for the moment. It isn’t clear that last week’s presidential Truth Social message about Chagos has been followed by a permanent change in US policy. But the Conservatives took the moment to deploy a successful spoiling motion in the House of Lords.

Our overall interests clearly lie in abiding by international law. That means continuing to recognise that Greenland is part of the Kingdom of Denmark, unless it chooses to become independent, and that the Chagos Islands should now belong to Mauritius, albeit with our leasing and subletting the Diego Garcia base. (This is also supported by the majority of Chagossians, who live in Mauritius, even if not by the smaller community living in Crawley in the south of England). Rejecting international law by seeing Greenland transferred to the US under duress and tearing up last year’s deal with Mauritius would be two more steps towards anarchy between the nations.

All responsible parties in British politics should stand together on this. When it comes to accepting that land can be taken by “power” and ignoring law, the stakes are even higher in relation to Ukraine. There it is getting increasingly hard to see differences of principle between Washington and Moscow. Britain and our close European and Commonwealth colleagues have an important and difficult hand to play. Having made their play in the Lords last Friday, the Conservatives should rejoin the other responsible parties in not weakening the Government’s difficult hand further.

• George Fergusson was the Governor of Bermuda from 2012 to 2016 and is now a retired senior British diplomat who writes on foreign affairs. This article has appeared in The Herald in Glasgow, and is reproduced by arrangement with it