Complex sentence: They suggested to Cao Cao to ally with Sun Quan and enlist his help in hindering Guan Yu's advances, and in return, Cao would recognise the legitimacy of Sun's claim over the territories in Jiangdong.
Split into simpler sentences: They suggested to Cao Cao to ally with Sun Quan and enlist his help in hindering Guan Yu's advances. In return, Cao Cao would recognise the legitimacy of Sun's claim over the territories in Jiangdong.