Complex sentence: In November 1942, the Japanese built an airfield on Engebi Island; because they used it only for refueling planes between Truk and islands to the east, no flying personnel were stationed there and the island had only token defenses.
Split into simpler sentences: In November 1942, the Japanese built an airfield on Engebi Island. As they used it only for refueling planes between Truk and islands to the east, no aviation personnel were stationed there and the island had only token defenses.