Complex sentence: By December, his unit moved to Finschhafen, where it could operate over the Solomon Sea against air forces of the Empire of Japan, and he quickly earned the nickname "Curly" because of his straight black hair.
Split into simpler sentences: By December, this unit moved to Finschhafen, where it could operate in the Solomon Sea against air forces of the Empire of Japan. Here, he quickly earned the nickname "Curly" because of his straight black hair.