Hi, just a different opinion on what Lu Wei said (please, don't kill the messager...lol). Politics and battles occure simultaneously not in a stagewise direction in RTK games...lol
Let's examine the politcal situations in ancient China which is unlike fuderal Europe when all nobles fighting each over to unified though conquest. Cao Cao's goal was to subjetation then, if nessary, by force.
the Zhang Lu in Hang Zhong and Liu Zhang in Yi province are currently fighting each other. By pointing your army to invade them, is hard to say there would not be a "Zhang Lu/ Liu Zhang" alliance. On top of that, you also have "Liu Bei/ Sun Quan alliance". so you are facing up to four separate force that could result similar as to "alliance of anti Dong Zhou".
if we assum, Cao Cao dealt a crushing blow to Liu and Sun at Chi Bi (the two biggest opposing forces at that time, but still, not that big). It will surely bring the unification sooner than just having to have wars against all four forces...
that just my opinion though, please post more if you have different view...lol
p.s. forgive my spelling errors. "Aghhhhh...."