Britain is bracing for a crushing defeat at the United Nations on Wednesday when the General Assembly is expected to adopt a measure demanding that London cede the British-ruled Chagos islands, home to an important military base.The Indian Ocean archipelago has been at the center of a decades-long dispute over Britain’s decision to separate it from Mauritius in 1965 and set up a joint military base with the US on Diego Garcia, the largest of the islands. The International Court of Justice in February handed Mauritius a victory when it said in a legal opinion that Britain had illegally split the islands and should give up control of the Chagos.After Britain rejected that ruling, Mauritius turned to the General Assembly to press for action. On Wednesday, the 193 countries of the assembly will vote on a draft resolution that welcomes the ICJ’s position and demands that Britain withdraw from the Chagos in six months.Diplomats expect the assembly to overwhelmingly back Mauritius in the vote, which is seen as a strong statement on decolonization. Britain cut off the Chagos islands from Mauritius before granting it independence in 1968 and evicted the entire population of islanders. Chagossians have been campaigning for their return for decades.It will be the second time in two years that Britain has to defend its ownership of the Chagos islands at the United Nations.In 2017, only 15 countries including Britain and the US voted to oppose a request for the ICJ ruling.Diplomats expect fewer countries to support the British stance in Wednesday’s vote.Embarrassing momentBritain has the support of the United States and is hoping that many countries — including European partners — will abstain to soften the blow.In 2017, 65 countries abstained including Germany, France, Spain and Canada, while 94 voted in favor of Mauritius’ push to seek a legal opinion.“The UK is going to lose,” said Richard Gowan, UN director for the International Crisis Group, who described the vote as “an embarrassing moment for the UK” as it seeks to show that it remains an influential global player post-Brexit.