Complex sentence: The main part of the war was waged by Ma Zhongying against the Xinjiang government, he was supported by Chiang Kaishek, the Premier of China, who secretly agreed to let Ma seize Xinjiang.
Split into simpler sentences: The main part of the war was waged by Ma Zhongying against the Xinjiang government. He was supported by Chiang Kai-shek, the Premier of China, who secretly agreed to let Ma seize Xinjiang.