The most basic rule of the military is to know your place, both figuratively (within the chain of command) and literally. If you're the squad leader, you know who your platoon leader (your immediate boss) is, and the company commander (his immediate boss) too. You also know who your team leaders (your immediate subordinates) are, and all the soldiers they are in charge of. If something happens to the platoon leader, one of the squad leaders will take charge of the platoon or the company commander will assign someone as the platoon leader. If something happens to a team leader, one of the soldiers will take charge of the fire team or the squad leader will assign one of them as team leader. A firm understanding of the chain of command allows the unit to keep fighting effectively even when suffering heavy casualties, because there is always someone in charge.
Looking at the literal side of things, soldiers knowing their place means there are no breaks in the camp perimeter for an infiltrator to sneak in through, and there are guards watching that perimeter to ensure that any attack is met with immediate resistance. In battle, there are no breaks in the line, and shields form walls or fields of fire overlap into kill zones. There are rules for who eats when, and how much, and there are rules for how to deal with equipment and how to deal with injuries and even, at times, when it is permissible to fight an enemy. These rules have existed in some form or another since the dawn of civilization, and the closer an army (or any other branch of the military) adheres to them and to the discipline that such rules imply, the more powerful that army is. The Roman army would march 20 miles in a day and build a fort at the end of the march, complete with walls and ditches, standardized streets and latrines. They would fight in three established lines, using tactics designed to build on their strengths and cover their weaknesses. Every soldier knew his place, and the Roman army regularly defeated armies of those who did not follow rules like these.
In fantasy RPGs, we often see a division of the game world into Law and Chaos. Those armies that most closely follow these types of disciplinary rules are by definition more Lawful. And in our fantasy games as well as in literature and movies, we see a common trope of "small band of Lawful characters hold out against large horde of Chaotic monsters". Sometimes, instead of a large horde of Chaos, there will be an equal or smaller number of Chaotic monsters, but each one is more powerful than a normal human. Such battles usually end with Law victorious, and RPG campaigns usually progress with Law expanding into the wilderness, name-level PC stronghold by stronghold.
Now, of course mere mortals rarely embody their own highest ideals, always falling short in some way. A Lawful dwarf sentry might just fall asleep at his post, and Chaotic gnolls are rarely as free as they'd like unless they're the strongest in the pack or willing to strike out on their own. But where mortal flesh is weak, the immortal spirits of the outer planes are mighty.
Everyone pretty much understands that the outer planes critters are more Good or more Evil than pretty much everyone on the material planes, but somehow this is often forgotten with regard to more Lawful or more Chaotic. I mean, everyone agrees that the Seven Heavens and Twin Paradises are oh-so-good and no one there would ever be impolite in any way, and everyone knows that the Nine Hells and the Abyss are nasty places, and when they think about it the fact that modrons are very Lawful and slaadi are very Chaotic isn't a problem for anyone, but I've heard many people make the argument that the legions of the Abyss could roll right over all other planes combined. To me this is completely forgetting the Law/Chaos aspect of alignment and the idea that denizens of the outer planes are exemplars of their alignment.
Sure, a horde of demons pouring from some vile portal would be devastating to the area they found themselves in, just from sheer numbers, but even at that, their Chaotic nature puts them at a disadvantage. I don't doubt that the Dukes of Hell could call up every devil not immediately necessary for the regular day-to-day operations of Hell, get them organized into ranks and files, move them through a portal efficiently, and set about taking over strategic points in the invaded land. If a powerful demon prince had a plan to open a portal capable of transporting a plane's worth of demons elsewhere, he almost certainly hasn't planned much past that. It doesn't even matter how intelligent he is; he can see all sorts of nuances in a situation and come up with all sorts of stratagems to take advantage, and he will still be too impulsive to fully capitalize on his own plan, much less herd all those Chaotic cats it would take to make it happen.
Oh yeah, big scary Demogorgon can whip them all into shape through fear...but Demogorgon can't be everywhere at all times, and when the cat's away the mice will play (to risk confusing cat metaphors). Those lesser demons only fear Demogorgon when Demogorgon is in their face, and when he's off scaring the piss out of the last bunch of deserters, they'll end up slaughtering each other over a few baubles. By the time he gets back to them, half will be dead and the other half will be scattered with their new-won treasures. The best plan a demon prince can carry out is to herd a mess of demons through the portal, ignore the fact that a lot of them get left behind, and then let them do their thing once on the other side. There will be no organized attack, just waves of disorganized yahoos going in all directions, not trying to take strategic positions, but trying to grab whatever catches their attention at any given time, and fighting among themselves as often as not. Sheer numbers will sweep away resistance at first, but an organized defense will stop them cold at some point, and they will be powerless to overcome it.
And that's why the Bloodwar of 2nd Edition AD&D is not an entirely lop-sided cakewalk for the Abyss. It was never a matter of nine planes versus 666, it was every bit of nine planes versus however many out of that 666 that happen to move in their direction at any given time, with the remainder working at cross purposes and in-fighting.
