AMSTERDAM (Reuters) - The World Court will issue an advisory opinion on Monday on whether Britain unduly pressured Mauritius to give up the Chagos Islands, which now houses a major U.S. military base, in exchange for independence in 1965.

The case at the International Court of Justice (ICJ), as the World Court is officially known, is seen as a test of whether colonial-era deals struck between great powers and small states were legitimate, given the power imbalance.

The Chagos Islands, now home to the Diego Garcia U.S. military base, is at the center of a bitter dispute between Britain and Mauritius. Mauritius said it was forced to give up the remote archipelago to gain independence during the decolonization process. Britain maintains the Indian Ocean nation gave up the Chagos islands willingly.

After gaining the islands in the early 1970s, Britain evicted almost 2,000 residents to Mauritius and the Seychelles to make way for the U.S. base.

For years Chagossians have dreamt of returning and have lobbied the UK government to let them do so. In 2016, Britain’s Foreign Ministry extended Diego Garcia’s lease until 2036, and declared the expelled islanders would not be allowed to go back.

While the ICJ’s advisory opinions are not binding, they carry great weight under international law as the ICJ is the United Nation’s highest court for resolving disputes.