Complex sentence: The coasts of Sardinia and Corsica were already under Carthaginian control when the city-state attempted, in three wars between 480 and 307 BC, to conquer Sicily, but were stopped by the Greeks, who had by then heavily colonized the island.
Split into simpler sentences: The coasts of Sardinia and Corsica were already under Carthaginian control when the city-state attempted, in three wars between 480 and 307 BC, to conquer Sicily. These attempts were stopped by the Greeks, who had, by then, heavily colonized the island.