Well,surprising no one,Exar Kun begs to differ.
I'd like to hear everyone's thoughts on this matter.My own?
I believe that Emperor Xian really did not add much to Wei.
The power of the Emperor was really limited to a few things:
1)Legitimacy.He who controls the Emperor can claim to be following a righteous cause.
Well this is all wrong at the start since the entire reason that the dynasty fell apart in the first place was because the regional leaders began to distrust the orders coming out of Luo Yang since the new Emperor was placed there by their biggest adversary.Really,sure you can shout it to the heavens and all that you're in the right,but who really cares?
Perhaps some might think that Xian would do some good as it came to recruiting officers but that won't be the case.Worthy minds would already know what's going on and would be joining to fight under the individual warlord rather than for the illusion of legitimacy of the Emperor.The only people who might be fooled are the peasants and really,they're just going to follow who runs their land anyway.
2)Other warlords are susceptible to the will of the Emperor.
This one is really a laugh.This stems from the supposed divine will of the Emperor and the power of an imperial edict.Well we already know that the warlords are fully prepared to accept edicts as they come...as long as the edict is giving them a promotion.
Titles.They accept titles from anybody who sounds legit.But anything other than that and they'll laugh in your face.If the Emperor was really so important to everybody why didn't they surrender to Cao Cao?It sure never affected the way people allied against him.
Once again the power of Xian seems to fail.
3)The Han dynasty.Controlling Xian givies one access to all the bureaucracy and giverning power of a 400 year dynasty.
Now I can't argue with this.I think that this would have a been a strong point for Cao Cao.Being able to have a ready made government for himself rather than having to try to set up some makeshift system to run things.No doubt Cao Cao benefitted greatly by having Han officials to run the affairs.
Course really,the question is,did these people even really count on Xian in the first place?
Course my answer is no.While the high officials will more than likely be loyal to a fault and be unwilling to carry out any sort of duties without the Emperor there,what of the more important people.?The mid level workers who make everything happen.
High officials can be replaced,but if there's a general strike you can't get anything done.I submit that these mid level workers would have been working due to their geographical location and would owe allegiance to their jobs rather than to a Son of Heaven.
If Cao Cao took Chang An without Xian in it I'm sure he'd still get an operational government to use for his own purposes.
So really I think that when Cao Cao took Chang An,the real prize he got wasn't some weakling named Liu Xie,it was in getting a city that had the old Han government tied to it.Notice that unlike other northern warlords,Cao cao actually seemed to have civil plans in place.Though his genius would have something to do with it,not having to set up his own governance would have helped immensely.
So,thoughts?

