This is a really good thread, and I've just been re-reading random sections of GoT:
Originally Posted by juleska
With respect to fighting an enemy in the more formal structure of a medieval battlefield, I believe Ned's wits were as strong the day he died as they were the day he helped Robert win the throne. I also don't think it's that he's incapable of thinking politically (there are plenty of instances and exchanges throughout the first book showing he has given thought to those things and sometimes acts correctly). I think it's that he so loathes the nature of the politics that he deliberately acts against them, as if to force the players to bend to the more honorable rules of the battlefield.
I noticed Robert Baratheon telling Ned about how keeping a throne is a 1000 times harder than winning one - implying a sense of naivety on the part of Ned.
Also, when Robert is dying, Ned presumes Cersei will fear and flee - both Renly and Littlefinger appear and advise Ned on prudent courses of action - neither of which he will follow up.
In fact, Littlefinger advises that if Ned pushes on Stannis claiming the throne, the seven kingdoms will fall to war as Stannis takes revenge against a whole host of houses, and warns there will be mass bloodshed.
However, Ned keeps steadfastly to "being honourable" to the point of not even being able to ask Littlefinger to buy the gold cloaks.
In this, I think it's suggested that Ned is used to being followed, and that as Lord of Winterfell he is used to dealing only with petty politics - and not grand schemes as in Kings Landing.
Ned uses "honour" to bind people to him, not least because he truly believes in it - but he uses it as a crutch, as I think Catelyn accuses him off in his decision to go to Kings Landing - because when faced with "hard" decisions, he falls back on a sense of "honour", which effectively takes any decision-making away from himself, and therefore the consequences - which become designated as "honourable" vs "dishonourable".
I think Jaime's later scenes show this up well - Ned always claims to follow "honour", and curses the "Kingslayer" as an oath breaker - yet did Ned not also swear an oath to follow and obey King Aerys?
For all his tactical ability in the field, and ability to think strategically, Ned is still very much the warrior, looking to bind people to follow him through common and - normally unchallengeable - ideals, such as honour.
However, in Kings Landing he comes across as something of a well-meaning country-bumpkin, unable to make the real and difficult political decisions that are necessary, and when faced with difficult choices, he abrogates his responsibily to do what is right and necessary by invoking "honour" as a simple moral backstop.
So, a great commander, when people are willing to follow, not least through aspiring to his ideals - but a poor politician when faced with real and hard decisions.
Heck, his decision to even go to Kings Landing he knew was a bad one - he was clearly warned about it, he clearly felt it - but saw refusal to accept the position of Hand as one that dishonoured both Robert and his family. He therefore willingly put them both in harm's way because he was naive enough to think that "right" would win through all.