Complex sentence: Japan occupied the islands in 1914 and received a League of Nations mandate over them in 1920, but after World War II the islands became trust territories of the United States, eventually gaining independence (1986/1994).
Split into simpler sentences: Japan occupied the islands in 1914 and received a League of Nations mandate over them in 1920. After World War II the islands became trust territories of the United States, eventually gaining independence (1986/1994).