Complex sentence: In 1914, during World War I, the island was captured by the Empire of Japan; she was awarded formal control of the island in 1918 by the League of Nations as a part of her mandated territory of Nanyo.
Split into simpler sentences: In 1914, during World War I, the island was captured by the Empire of Japan. The Empire was awarded formal control of the island in 1918 by the League of Nations as a part of its mandated territory of Nanyo.