Complex sentence: Zhang He predicted that Zhuge Liang's army lacked supplies so they cannot maintain the siege on Chencang for long, so he replied, Zhuge Liang will have already left before I even reach Chencang.
Split into simpler sentences: Zhang He predicted that Zhuge Liang's forces could not maintain the siege on Chencang for long because they lacked supplies. He replied, Zhuge Liang will have already left before I even reach Chencang.