StreamOfTheSky
Adventurer
From this question: Would any commander worthy of the name ever assume the wizard was on his own? The commander would allow his forces to be spread so thinly that he cannot communicate with them and allow them to be cut apart by cavalry?
Probably not.
The defense suggested is only useful in a very specific circumstance - there is only one wizard, and the army knows there's only one wizard. That in no way settles the question in general.
Look, I know that in real life your points are true about infantry formations. But what about in the D&D rules? Set vs. Charge can be pretty viscious, and I don't see many inherent advantages in being tightly grouped against cavalry. I suppose preventing them from riding past into the clear (ride by attack) by having people behind you is useful, as is the likely greater amount of AoO's the cavalry would suffer with the infantry close together. But...those don't seem like such major benefits as to make it suicidal to stagger the formation, purely by the 3E rules....
From the wikipedia entry I linked to before, it notes:
"In typical military strategy, the success of any cavalry charge depends on the infantry breaking ranks so that the cavalry can mow them down. If the infantry does not break, however, cavalry charges will often fail - with even trained warhorses refusing to advance into the solid ranks of opponents."
Now the very last part supports what you say. But there are no rules for horses for such things. Furthermore, there are no morale rules (Heroes of Battle, made for this stuff, does have them, admittedly), so by RAW the infantry simply won't break. I know it says all over the place that this wouldn't be the case (like in the Warfare section in Complete Warrior, stating the most common soldier is a conscripted commoner who'll run away easily), but it's still rather vague in the rules about how easy it is to break their morale.
I just...I don't even know if cavalry charges should be as effective in D&D as they were in real life. The charge rules already make a knight with a lance more devastating than he probably should be. And there aren't many "free" group fighting tactical benefits in even the supplements to support a tightly packed infantry. Even the Shield Wall is not free to use, but a Tactical Feat, iirc.
Well, if they're travelling anywhere they'll be on a road _ *boom*. Or they're drawn up facing an army allied with the wizard *boom*. It only takes a small army in close order to beat a large army that's had to disperse. Likewise besieging a town, castle, or most other military tasks (guerilla tactics are still efffective though)
It is however true that if all you are trying to do is survive, the Wiz will have trouble killing you. The issue is more that 200 troops + Wizard beats 2,000 troops with no Wizard. In fact I've run an awful lot of battles just like that - PCs plus a small regular force outnumbered 10:1 by the Orc Horde; PCs fly over the orcs and zap them, regulars mop up.
Sounds like Agincourt now.

Replace all that focused power that's in the wizard with it spread out evenly among the entire small force, and you basically have any real life battle situation where the smaller side has superior technology or skills ideal for that battlefield. And yeah, a Wizard and some regular guys are stronger than the sum of their parts. Yet another reason why I say, in a world with magic, if one side has a significant edge over the other one in that regard, the "low tech" side will need great numbers, tactics, and/or luck to overcome that deficiency. While your occasional story of a wizard led elite unit crushing a small army can happen, more often than not, if the enemy is aware of their magic disadvantage, they would delay hostilities until they could do something about it. Even if you have enough soldiers to beat the wizard's forces, it'd probably be a rather Pyrrhic victory most commanders would not want to obtain (whilst ones like Xykon who don't care if his own people die would not hesitate to accept).
You've yet to convince me how a spellcaster lower than high levels (~14-16 and up, hard to quantify) left without any worthy opposition on the other side could cause more devastation than in actual historical conflicts where the smaller side won a crushing victory. ...Or took a lot of folks to hell with them.