Mark Langsdorf is running a fantasy Mass Combat based game. It's good and interesting reading. I'm reading it with great interest because I'd like to use Mass Combat in my gaming without forcing my players to play hex-and-counter or minis-and-rulers wargaming if they don't want to.
On his latest post, though, I put up a comment and realized I'd like to expand on it here.
"But if you don't have wizards and your opponent does, you'll lose even if you're the better general with more troops."
I find that to be true in tactical fights in GURPS, using GURPS Magic - there are too many "I can do this to you, and you can't stop me and can't recover from it" effects to magic. It doesn't take a lot of magic on one side to make it a decisive advantage, especially if the other side has none. That might not fit your game, but it does sound about right - one side has magic, one side does not, so the latter side loses the vast majority of the time. It's why, in my Felltower game, I made sure to spread magic around freely. Orcs and gnolls and so on don't have 1-2 spellcasters per 30-300 tribe members, they have dozens of minor workers so the PCs wizards need to counter them.
The in quotations marks line is his, the rest is mine. He's using a different magic system, so the rest of this commentary is not meant to imply anything about or apply to Mark's game in any way, shape, or form. I'm springboarding, here, not criticizing. I'm saying this so emphatically because I don't want to argue magic systems or criticize a clearly excellent campaign, I want to share my experiences with magic vs. no-magic in my game and discuss that.
I've mentioned this before - the whole "wizards, not shamans" bit.
I find that if you've got a proper mix of spells for a situation - which often means one well-chosen spell - you can nearly assure yourself of a victory.
Foe depends on missile weapons? Missile Shield.
Foe is undead? Turn Zombie, True Faith (with Turning).
Foe has low damage? Armor spell.
Foe has low skill? Blur.
Foe can't see in the dark? Blackout plus Dark Vision.
Foe has no reach and is vulnerable to missiles? Levitation or Flight.
Foe has a low Will? Any Mind Control spells, especially Madness.
Foe depends on close-in coordination? Spark Storm, Windstorm, Rain of Stones, Explosive Lightning or Explosive Fireball, etc.
It goes on and on from there. It changes if the foe has some magic support. Even a little bit of magic goes a long way. The difference between magical supremacy (magic vs. no magic) and magical superiority (magic vs. weak magic) is noticeable. The enemy might not be able to stop your magic with his magic - even with easier counterspelling - but he can apply his well-chosen spells to counter your advanages. Now your low-Will allies are vulnerable to Madness. Your melee fighters can't reach him without their own Levitation or Flight. Your lower-damage skilled guys can't penetrate his Armor spell. Your side-by-side "block for each other" fighter types are caught in Explosive Lightning blast radii when they throw them.
What often happens is the wizards start to angle to take out the other wizards - not only keeping them from influencing the fight against you, but effectively keeping you from influencing the fight against them until you pull off your plan. The foe having wizards tempts the "bust our fast guy through and have him solo charge the mages!" strategy, which can be fatal (or at least take your best guy out while he tries.)
So yes, in my experience, magical supremacy is huge. It can be decisive even with inferior numbers. It's like being the only side in a fight with air support or artillery or firearms, only more so because so many effects can be literally unstoppable or without effective mundane counters.
GM Takeaway - Put lots of magic-using foes in, even minor ones, to effectively challenge DF parties in particular but any spellcasting fantasy PCs in general.
Player Takeaway - Don't depend on magical supremacy to win fights. Enjoy it when you get it, but don't panic when it's not there no expect it to be the basis of all of your combats. Bring as much magic as you can, learn to use it well, and know that it's a two-way street.