Complex sentence: In 1959, Peng Dehuai wrote a letter to Mao, in which he compared himself to Zhang Fei: because of Mao's popular association with Cao, Peng's comparison implied that he had an intuitively confrontational relationship with Mao.
Split into simpler sentences: In 1959, Peng Dehuai wrote a letter to Mao, in which he compared himself to Zhang Fei. Because of Mao's popular association with Cao Cao, Peng's comparison implied that he had an intuitively confrontational relationship with Mao.