Complex sentence: The initial cause of the conflict was over the control of Shanghai, China's biggest city and port, which was legally a part of Jiangsu province under the control of Zhili general Qi Xieyuan.
Split into simpler sentences: The initial cause of the conflict was over the control of Shanghai, China's biggest city and port. It was legally a part of Jiangsu province, which was under the control of Zhili general Qi Xieyuan.