Complex sentence: The Ryukyos extend southwest from Kyūshū (the southwesternmost of Japan's main four islands) to Taiwan, however Okinawa Prefecture itself encompases only the southern two thirds of that chain.
Split into simpler sentences: The Ryukyos extend southwest from Kyūshū (the southwesternmost of Japan's main four islands) to Taiwan. The Okinawa Prefecture encompases the southern two thirds of that chain.