Complex sentence: Part of this army was the Sixth Infantry Division, which was considered Japan's best division in the early Chinese campaigns and played a major part in the Rape of the Chinese city of Nanking in 1937.
Split into simpler sentences: Part of this army was the Sixth Infantry Division, considered Japan's best division in the early Chinese campaigns. It played a major part in the Rape of the Chinese city of Nanking in 1937.