Officials from the Indian Ocean island nation of Mauritius told United Nations judges Monday that former colonial power Britain strong-armed its leaders half a century ago into giving up territory as a condition of independence, a claim that could have an impact on a strategically important U.S. military base.

Judges at the International Court of Justice began hearing arguments for an advisory opinion the U.N. General Assembly requested on the legality of British sovereignty over the Chagos Islands. The largest island, Diego Garcia, has housed the U.S. base since the 1970s.

"The process of decolonization of Mauritius remains incomplete as a result of the unlawful detachment of an integral part of our territory on the eve of our independence," Mauritius Defense Minister Anerood Jugnauth told judges.

Mauritius argues that the Chagos archipelago was part of its territory since at least the 18th century and taken unlawfully by the U.K. in 1965, three years before the island gained independence. Britain insists it has sovereignty over the archipelago, which it calls the British Indian Ocean Territory.

Jugnauth testified that during independence negotiations, then-British Prime Minister Harold Wilson told Mauritius' leader at the time, Seewoosagur Ramgoolam, that "he and his colleagues could return to Mauritius either with independence or without it and that the best solution for all might be independence and detachment (of the Chagos Islands) by agreement."

Ramgoolam understood Wilson's words "to be in the nature of a threat," Jugnauth said.

British Solicitor General Robert Buckland described the case as essentially a bilateral dispute about sovereignty and urged the court not to issue an advisory opinion.