I think there are several reasons not to use hordes of low-level armies for armies.
1) They can't kill much. They are severely underclassed against even moderately high CR monsters or leveled NPCs. They are virtually useless against anything with flight, fear effects, area attacks, or damage reduction.
2) They are hideously expensive. Ok, let's assume that you have a professional army of 10,000 people. In just one year, it will cost 1,825,000gp to feed them all. If you want to actually go on the offensive, it will take quite a bit of logistical support to sustain those 10,000 troops. We're talking about needing to haul 10,000 lbs of food to your army every day. The farther you wish to go (i.e. the longer the supply line is), the more wagons you'll need. And then you need to worry about clean water, which could be handled with several decanters of endless water.
You also need to equip those soldiers. Say something like each soldier is equipped with scale mail, large wooden shield, longsword, possibly some javelins. Call it 75gp per soldier. That's another 750,000gp just to equip those 10,000 soldiers with mediocre equipment. Archers are more expensive (bows are quite expensive), and cavalry even more so (even light warhorses are 150gp, then there's the saddle, bit and briddle, feed...)
And THEN, those 10,000 soldiers could be doing something productive instead of going off to war. Farming, blacksmithing, etc.
3) They are incredibly vulnerable. Disease, area attacks, "dreadnaught" units, fear, poor morale, starvation etc. are all ways to easily knock out mass amounts of low-level warriors at little risk. And then, you know, you only have so many people in your kingdom. The people won't like it for very long if you keep losing tens of thousands of soldiers in your campaigns. Even then, you'll eventually either run out of a) people willing to go to war or b) people physically able to go to war.
It seems to me that low-level warriors are good for one thing: controlling the peasants. In which case those soldiers would be an occupying force, and not an invading force, to be used only when you have taken out your enemy's major combatants (high level PCs, golems, summoned creatures, basically anything with CR > 6 or 7).
In such a case, I would think that "wars" would really consist of opposing strike forces, consisting of high level characters and high CR monsters, well-equipped with magic items, duking it out. In the same way that air forces can battle for air superiority, high level NPCs would seek to battle for magic superiority. When one side is sufficiently beat down, a treaty could be signed or the winning side could actually invade with their modest army of low-level warriors (after, of course, the enemy's low-level army is ground into dust by your remaining high-quality troops).
I would imagine that in each major city/region, one would have a major "safehouse" for all of the powerful army elements in the city. These safehouses would be chook full of magical wards and defenses, to protect against incorporeal creatures, teleporting enemies, etc. Likely, each major city would be (probably through portals) connected to each other, allowing instantaneous reinforcing if one comes under attack.
Attacks would likely consist of one side teleporting to an enemy city to achieve some goal (attacking an enemy safehouse, destroying city infrastructure, killing enemy garrisons, assassinating the governor/nobles, stealing precious diamonds, etc.) There would be losses on both sides, but both sides could also reasonably raise their dead. So, in reality, the war of attrition becomes 'how many enemy levels can I take out' rather than 'how many War1s can I take out'. Connected to that, when an enemy dies, it presents an excellent opportunity to steal his magic items for your own use, so that even if the enemy is raised, he won't have any of his gear and will thus be quite a bit weaker. Or, hey, just steal your enemy's stockpile of diamonds to reduce his ability to raise his fallen soldiers.
Of course, all of this is limited by how many real quality NPCs/monsters there are in the employ of each kingdom. I'd imagine in a medium size kingdom (say 1-2 million inhabitants) you'd have a few dozen on each size, with a mundane army of low-level Warriors numbering maybe 10,000-20,000.
One advantage of this system is that it is directly useful from a PC's perspective. Squads of high-level, well equipped soldiers/monsters running around killing other high-level, well equipped soldiers? That's PC material if I ever saw it. In this case, your wars are centered around squad-sized, short, viscious engagements with tons of magic on each side.
Who needs boring rules for how many War1s you killed with your fireball, or how many wagons and horses it will take to supply 26,529 soldiers with food and water daily, or why those War1s would even bother staying in the fight when they've just seen 3,000 soldiers be incinerated/eaten by terrible monsters/run away from dragon fear/be crushed by an iron golem's mighty fist/be hewn down by a 15th level fighter with a spiked chain, great cleave, and combat reflexes.
Let's face it, even if you have 20,000 Warrior 3s, they're no match for 4 level 20 characters. Even with a few scattered level 7-9s, they'd eventually be worn down in any battle of attrition. The only counter to high quality combatants ("dreadnaughts") is your own, in which case, why even have mooks around in the first place? To kill the other sides' mooks? Maybe, but why bother risking your thousands of well-trained soldiers when you can just wait until your opponents' big combatants are killed by your own? There's a reason Hitler wanted air superiority before he invaded Britain; because otherwise his invading force is vulnerable to that air force and will take serious losses as a result. Unless you want to lose thousands of your soldiers at a time, there's no reason to send them into an incredibly hostile environment unless you absolutely had to. To use another WWII analogy, towards the end of the war, Germany had practically no air force remaining. They kept fighting with their conventional forces, but those forces got pulverized whenever they tried to move anywhere.
I don't know, to be honest, I haven't really thought of this that much. Are there any major flaws in this plan? Anything major that I'm totally forgotting? Of course, this assumes quite a high level of magic, but I think the point stands in any setting which will have even a moderate amount of 'things' out in the world with high CR.
1) They can't kill much. They are severely underclassed against even moderately high CR monsters or leveled NPCs. They are virtually useless against anything with flight, fear effects, area attacks, or damage reduction.
2) They are hideously expensive. Ok, let's assume that you have a professional army of 10,000 people. In just one year, it will cost 1,825,000gp to feed them all. If you want to actually go on the offensive, it will take quite a bit of logistical support to sustain those 10,000 troops. We're talking about needing to haul 10,000 lbs of food to your army every day. The farther you wish to go (i.e. the longer the supply line is), the more wagons you'll need. And then you need to worry about clean water, which could be handled with several decanters of endless water.
You also need to equip those soldiers. Say something like each soldier is equipped with scale mail, large wooden shield, longsword, possibly some javelins. Call it 75gp per soldier. That's another 750,000gp just to equip those 10,000 soldiers with mediocre equipment. Archers are more expensive (bows are quite expensive), and cavalry even more so (even light warhorses are 150gp, then there's the saddle, bit and briddle, feed...)
And THEN, those 10,000 soldiers could be doing something productive instead of going off to war. Farming, blacksmithing, etc.
3) They are incredibly vulnerable. Disease, area attacks, "dreadnaught" units, fear, poor morale, starvation etc. are all ways to easily knock out mass amounts of low-level warriors at little risk. And then, you know, you only have so many people in your kingdom. The people won't like it for very long if you keep losing tens of thousands of soldiers in your campaigns. Even then, you'll eventually either run out of a) people willing to go to war or b) people physically able to go to war.
It seems to me that low-level warriors are good for one thing: controlling the peasants. In which case those soldiers would be an occupying force, and not an invading force, to be used only when you have taken out your enemy's major combatants (high level PCs, golems, summoned creatures, basically anything with CR > 6 or 7).
In such a case, I would think that "wars" would really consist of opposing strike forces, consisting of high level characters and high CR monsters, well-equipped with magic items, duking it out. In the same way that air forces can battle for air superiority, high level NPCs would seek to battle for magic superiority. When one side is sufficiently beat down, a treaty could be signed or the winning side could actually invade with their modest army of low-level warriors (after, of course, the enemy's low-level army is ground into dust by your remaining high-quality troops).
I would imagine that in each major city/region, one would have a major "safehouse" for all of the powerful army elements in the city. These safehouses would be chook full of magical wards and defenses, to protect against incorporeal creatures, teleporting enemies, etc. Likely, each major city would be (probably through portals) connected to each other, allowing instantaneous reinforcing if one comes under attack.
Attacks would likely consist of one side teleporting to an enemy city to achieve some goal (attacking an enemy safehouse, destroying city infrastructure, killing enemy garrisons, assassinating the governor/nobles, stealing precious diamonds, etc.) There would be losses on both sides, but both sides could also reasonably raise their dead. So, in reality, the war of attrition becomes 'how many enemy levels can I take out' rather than 'how many War1s can I take out'. Connected to that, when an enemy dies, it presents an excellent opportunity to steal his magic items for your own use, so that even if the enemy is raised, he won't have any of his gear and will thus be quite a bit weaker. Or, hey, just steal your enemy's stockpile of diamonds to reduce his ability to raise his fallen soldiers.
Of course, all of this is limited by how many real quality NPCs/monsters there are in the employ of each kingdom. I'd imagine in a medium size kingdom (say 1-2 million inhabitants) you'd have a few dozen on each size, with a mundane army of low-level Warriors numbering maybe 10,000-20,000.
One advantage of this system is that it is directly useful from a PC's perspective. Squads of high-level, well equipped soldiers/monsters running around killing other high-level, well equipped soldiers? That's PC material if I ever saw it. In this case, your wars are centered around squad-sized, short, viscious engagements with tons of magic on each side.
Who needs boring rules for how many War1s you killed with your fireball, or how many wagons and horses it will take to supply 26,529 soldiers with food and water daily, or why those War1s would even bother staying in the fight when they've just seen 3,000 soldiers be incinerated/eaten by terrible monsters/run away from dragon fear/be crushed by an iron golem's mighty fist/be hewn down by a 15th level fighter with a spiked chain, great cleave, and combat reflexes.
Let's face it, even if you have 20,000 Warrior 3s, they're no match for 4 level 20 characters. Even with a few scattered level 7-9s, they'd eventually be worn down in any battle of attrition. The only counter to high quality combatants ("dreadnaughts") is your own, in which case, why even have mooks around in the first place? To kill the other sides' mooks? Maybe, but why bother risking your thousands of well-trained soldiers when you can just wait until your opponents' big combatants are killed by your own? There's a reason Hitler wanted air superiority before he invaded Britain; because otherwise his invading force is vulnerable to that air force and will take serious losses as a result. Unless you want to lose thousands of your soldiers at a time, there's no reason to send them into an incredibly hostile environment unless you absolutely had to. To use another WWII analogy, towards the end of the war, Germany had practically no air force remaining. They kept fighting with their conventional forces, but those forces got pulverized whenever they tried to move anywhere.
I don't know, to be honest, I haven't really thought of this that much. Are there any major flaws in this plan? Anything major that I'm totally forgotting? Of course, this assumes quite a high level of magic, but I think the point stands in any setting which will have even a moderate amount of 'things' out in the world with high CR.