Complex sentence: In 1989, Nauru took legal action against Australia in the International Court of Justice over Australia's administration of the island, in particular Australia's failure to remedy the environmental damage caused by phosphate mining.
Split into simpler sentences: In 1989, Nauru took legal actions against Australia in the International Court of Justice over Australia's actions during its administration of Nauru. In particular, Nauru made a legal complaint against Australia's failure to remedy the environmental damage caused by phosphate mining.