Subodai, arguably the greatest general the world has known, was carried into battle on a litter. Napoleon was known as "The Little Corporal" and I may be wrong but I don't recall evidence that he was a great hand to hand fighter. Wellington was a notoriously poor shot. I see little evidence that any of these men was a fighter. Yes, they could wield weapons. But that was about it. And classes other than fighters can wield weapons - so why insist that generals need to be fighters? There is, so far as I can tell, little mechanical support for this.
And on the other hand there are people who are fighters. The general's bodyguards. Very few generals will have been better fighters than their bodyguards. Which means the bodyguards are better fighters than the general - but somehow the general needs to be a higher level fighter than his bodyguards to be a general?
And if you look mechanically, so far as I can tell in all the skills required for generalship the bard is at least as good as the fighter - better at inspiring his troops, better at knowing the mind of his people and the enemy generals, better at knowing the terrain. And a bard with Perform (Oratory) gives great speeches. He just gets beaten round the training ring by his bodyguards (as you'd expect).
So why do you insist that the general should be a fighter? I see no scrap of mechanical support for this. And a lot of mechanics indicating that bards and clerics both do a better job.
Well, the querstion began with how does literature address the imbalance between warriors and wizards (bards and clerics just make it painful).
I see a number of ways:
1) Magic doesn't work well in combat (see Fred Saberhagen's Empire of the East) so warriors are required for fights
2) Magic is slow, subtle, and hard to use so it is not suitable for flashy use on the battlefield (Icelandic Sagas seem to fit into this category)
3) Magic isn't all that powerful so a wizard can do neat things but so can a tough warrior (Gandalf in the Hobbit seems to fall into this category)
4) Warriors have influence on the world via leadership skill and/or social status that make them able to do things mages cannot (Dragon Age and King Arthur are examples of this trope)
5) Wizards have some sort of special vulnerability or limitation that makes them rely on warriors (classical folktales are filled with these)
6) Warriors can do supernatural feats or are inherently magical (Tomas from the riftwar, Manga, Achilles)
I kind of like #4 as a way of balancing high level characters. Making the cleric better at #4 than the fighter (at the levels where clerics are hardly weak at melee) seems to be an odd way to balance the classes.