Complex sentence: About 17,000--19,000 Chinese and 15,000 Japanese were killed in the resulting battle, the higher number of Chinese casualties caused by the time that the Japanese had to prepare their fortifications on the south side of the river.
Split into simpler sentences: About 17,000--19,000 Chinese and 15,000 Japanese were killed in the resulting battle. Tthe higher number of Chinese casualties was caused by the fact that the Japanese had time to prepare their fortifications on the south side of the river.