Complex sentence: At a costume party he attends with his parents, he wanders off and encounters a Japanese airplane that had been shot down, nearby, he finds a camp full of Japanese troops in a trench and is taken aback.
Split into simpler sentences: At a costume party he attends with his parents, he wanders off and encounters a Japanese airplane that had been shot down. Nearby he finds a camp full of Japanese troops in a trench and is taken aback.