Complex sentence: Yan Xishan continued in his attempts to work with both sides, creating the impression among Li's supporters that he was a "stooge" of Chiang, while those who supported Chiang began to bitterly resent Yan for his willingness to work with Li.
Split into simpler sentences: Yan Xishan continued in his attempts to work with both sides, creating the impression among Li's supporters that he was a "stooge" of Chiang. Those who supported Chiang began to bitterly resent Yan for his willingness to work with Li.