It’s all to do with nuclear weapons and the African Nuclear weapons free zone which came about in 1996.
_"In early 2023, the United Kingdom and Mauritius renewed negotiations to settle the Diego Garcia sovereignty dispute. Noting the significance of the U.S. base on Diego Garcia, Mauritius had previously offered the United States a 99-year lease on the island in exchange for full sovereignty over the Chagos Archipelago.
However, the offer included an explicit commitment that Mauritius support the “unhindered operation” of the U.S. military base, reviving outstanding questions about the inclusion of Diego Garcia in the African nuclear weapons-free zone.
The proposed lease thus raises an underlying tension in the sovereignty negotiations that challenges the principles of American and British nuclear nonproliferation. Under the current terms, the nuclear status of Diego Garcia must be settled before the sovereignty dispute can be addressed.
Last December, the U.K. dropped the negotiations, citing government concerns that ceding Chagos to Mauritius would endanger U.K. and U.S. defense interests in the Indo-Pacific. At present, the U.S. lease of Diego Garcia from the U.K. is set to expire in 2036_."
