I'll play devils' advocate to you, Hammerhead. I, too, try to find how things would evolve under (most) of the assumptions of the game, but think of these things...well, first a little quote from the founder of the game:
"...Use the written material as your foundation and inspiration, then explore the creative possibilities you have in your own mind to make your game something special...IT IS THE SPIRIT OF THE GAME, NOT THE LETTER OF THE RULES, WHICH IS IMPORTANT. NEVER HOLD TO THE LETTER WRITTEN, NOR ALLOW SOME BARRACKS ROOM LAWYER TO FORCE QUOTATIONS FROM THE RULE BOOK UPON YOU, IF IT GOES AGAINST THE OBVIOUS INTENT OF THE GAME. AS YOU HEW THE LINE WITH RESPECT TO CONFORMITY TO MAJOR SYSTEMS AND UNIFORMITY OF PLAY IN GENERAL, ALSO BE CERTAIN THE GAME IS MASTERED BY YOU AND NOT BY YOUR PLAYERS. WITHIN THE BROAD PARAMETERS GIVEN IN THE ADVANCED DUNGEONS & DRAGONS VOLUMES, YOU ARE THE CREATOR AND FINAL ARBITER, BY ORDERING THINGS AS THEY SHOULD BE, THE GAME AS A WHOLE FIRST, YOUR CAMPAIGN NEXT, AND YOUR PARTICIPANTS THEREAFTER, YOU WILL BE PLAYING ADVANCED DUNGEONS & DRAGONS AS IT WAS MEANT TO BE..."
ok, may not all that relevant to this thread but it contains the seeds of much rpg wisdom, grasshop...um, everyone....
ANYTHING is supported within the D&D rules framework. ANYTHING. You just modify as you please.
OK...to go on to the true response rather than letting my autism take over...
Hammerhead...think of this...
Your party teleports in and starts doing their darndest to kill the army. Yes, they're invisible, but detect invisibility is a low level spell. What happens when a simple spell like hold person or charm person is cast? Or a more powerful version? It takes very few low level priests to take out a POWERFUL party when they are layering hold person spells. If the party consists of 6 12th level people, then the other army will also probably have that number, all else being equal. Or they'll have a hoard of lower-level guys...or all of the above.
There's no denying the sheer power that a high-level party can bring to bear, but all that has to happen is for a each of them to fail a single saving throw (or the 3E equivalent). Not even all at once. Think of Steven Brust's books...although raising the dead is possible, they still do things like cutting off the heads of the slain so that raising them is more difficult. Simple spells and strategies can bring any party down. After all, the PCs usually end up doing something to defeat enemies that would, in open combat, slaughter them. The enemy army is much the same.
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My own take on things (hopefully I haven't posted this to this thread...I know I have to some thread on this or another rpg board)...
The spells that a lot of people think are the spells of a "war wizard" are really not all that effective in the big scheme of things. Yes, a fireball will roast low level people but how many can it actually get on an open battlefield? Not many. Cloudkill is great, too, but how many will it actually effect? Not a lot. Most of these spells are far more effective in a dungeon or other enclosed space than on a battlefield.
What, then, do I think are the most effective spells? Charms and illusions, hands-down.
Why? Because charms get people on your side. We all know that it's not what you do but who you know that matters in life. The same in the rpg world. Long-term spies who are magically skilled can take plenty of time and make a lot of charmed "friends" who can and will leak plans to them, give them favors, and tell them what is what. "Knowing is half the battle", after all. Even a 1st level guy with a charm person spell can influence many things and many people. Of course, in keeping with what I've said above about counters to the 12th level party, there will be someone who eventually wises up to the charms and starts screening or protecting crucial people more. It's like the arms race. Still, though, charms are among the most potent spells in the game. Even in combat, a spell that automatically takes an enemy out of the battle with a missed save, no matter how they're out of the battle, is one of the best things around.
Illusions are another awesome category. In battle, what is PERCEIVED is more important than what IS. Even low level casters with illusions can make more of a difference than a guy flinging fireballs and lightning bolts (although the morale effects can be pretty nice with stuff like that). HOWEVER...even a simple illusion that takes nothing to do and isn't flashy in the least can make a HUGE difference in a battle. For example...if someone casts and maintains an illusion of even a relatively shallow ditch in the middle of the battlefield, a troop of enemy cavalry SHOULD avoid it during a charge since it would slow their momentum. That cavalry charge that was turned aside might have shattered your front lines. Even something more obvious, like a barrage of arrows falling into enemy ranks will cause morale problems and disorder. Although heroic figures might disbelieve some illusions, the common soldier will have a hard time with it if you tell him those arrows aren't real and he shouldn't worry about them...especially if you mingle a few real arrows in with the fake ones. Disguise possibilities for spying is another facet of this as is the maintenance of the illusion of a lot more campfires at night than actually exists.
Something that I didn't mention is divination spells. Properly prepared and used, divination spells are AWESOME in what they can do. As I said before, "knowing is half the battle".
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An example from my own world, though years ago when I wasn't as frugal with magic as I am now....
The party was high level. The mage/leader was an 18th level alteration specialist. Another character was a 14th level druid. Another was a 15th level ranger, another a 13th level fighter (with a rod of flailing, girdle of giant strength, and flail specialization, if that means anything to those of you who are aware of what that combination of items and abilities could potentially do in combat), and the last was a fighter/magic-user/thief of about 10th level in the first two classes and 12th level in the last. This group was trying to unite a group of small, feuding kingdoms into one force to fight a conqueror who was taking all of them over one by one. The group rather arrogantly began to do just the sort of tactics that Hammerhead espoused and they worked well for a while. So well that the players thought that they would just forget about uniting the kingdoms and do everything themselves.
Well, things didn't happen that way...
The PCs were paranoid and never met with anyone else other than themselves to plan things out, even though they met in the castle of one of the lords whose lands they were defending. After many casual questions, I found that they didn't pay much attention to their surroundings while they were there, thinking of big foes rather than little spies. Well, one of the assassins employed by the enemy happened to have a hat of disguise and a medallion of ESP. She penetrated the castle without a problem and ended up taking the role of a servant girl (mostly) or a guard posted outside of the "war room". With the medallion, she gained A LOT of knowledge about the plans of the PCs. She stayed in place and relayed information to her employers.
The PCs ended up with more victories and the enemy watched and made sure that the information that they were getting was always correct. While doing so, they marshalled their own resources.
Mages and priests with spells such as charm person, blindness, detect invisibility, phantasmal force (and various improved versions), invisibility, ray of enfeeblement, stinking cloud, dispel magic, hold person, slow, suggestion, confusion, emotion, enervation (this one really hurts a high level spellcaster), improved invisibility, polymorph other, command, entangle, heat metal, and silence were arrayed against them. Due to the arrogance of the party leader in saying that no one of low level could possibly hurt them, I made sure that no one in the enemy party could use spells greater than 4th level.
The PCs were given a hint through their own scouting that a major force was building up to attack a certain town. Pickings had been easy thus far and they weren't all that careful in the preparations. The spy caught a lot of their plans and everything was in readiness for them when they went in. Their plan was to attack the enemy commander in his tent at night and then lay waste to the camp.
Well, they teleported in (teleporting is, in itself, a risky business because the odds are that, if used a lot, the party will get hurt seriously (remember that in 2E and 1E, you didn't just take damage from a "low" teleportation result but would end up teleporting into solid ground and likely dying rather than just taking some damage like in 3E). Anyway, they came in with "guns blazing" so to speak and were met by a barrage of spells from the invisible mages and priests in the area. The PCs got the first attacks because they were all invisible but most became visible after attacking. A barrage of hold person spells took a couple of them down (rememer that, in 1E and 2E, hold person spells can affect multiple targets and the fewer targets the more penalty on their saving throws) because even high level characters can't make ALL of their saving throws all of the time. Certain of them were hit by multiple dispel magic spells, dispelling protections.
The PCs were greatly shocked at this. They started turning their attentions toward the mages and priests but there were about three times the number of party members, so the PCs split up. Even in the next round, the havok that they wrought upon the relatively low-level foe was enormous...but then the foe got to attack. More hold person spells were cast. Command spells, confusion, stinking cloud, more dispel magics...the 18th level mage ended up getting his improved invisibility spell dispelled and being hit by enervations spells from three different guys (only one actually got him but the resulting two levels drained for a few hours along with random spell loss screwed up his carefully-chosen spell list). There were only two PCs left standing at this time...the 18th level mage and the multiclassed guy. Both were quite shaken at this rapid turn of events and there was a lot of swearing going on toward me. HA!
They had a parting of ways because they had different ideas on how to handle stuff. The 18th level mage teleported away because he thought discretion was the better part of valor and he figured he could get his comrades resurrected later (they had died many times over the years). The multiclassed guy thought that victory was within their grasp because of the havoc they had already wrought and fought. He actually did quite well for a couple of rounds while making all of his saving throws and fighting with weapons rather than spells. Then, of course, he missed a saving throw and was taken out of commission.
I could have just had the enemy dispose of the corpses in a fire and have done with everything but, wanting the story to continue, the 18th level mage gathered together some followers and came back later in the night and rescued his comrades.
A lot of low level spells can get through just about any defense. If someone is immune to the lower-level stuff because of their defenses, then it is a simple matter to send in higher level casters. If all else fails, a spellcaster with antimagic shell accompanied by a couple of bad-ass fighter-types can do a lot of damage to people used to having magical protections up and going. Or, alternatively...who needs bad-ass figher-types? Antimagic shell and some archers with poisoned arrows can also do wonders to a high level mage who thinks that he's invulnerable.
Sorry for the long post...just trying to throw out some ideas...