Blood War

In the Planescape campaign setting for the Dungeons & Dragons fantasy role-playing game, the Blood War is the eternal war of fiends fought between the Tanar'ri, the chaotic demons of The Abyss, and the Baatezu, the lawful devils of Baator, with the neutral evil Yugoloths playing both sides against the other.

The war has raged on since before most worlds on the Prime Material Plane even existed. No one is certain of the exact causes of the war, though the Neutral Evil fiends, the Yugoloths, view the Blood War as their own personal experiment into the nature of evil, one which they themselves planned from the outset. What is certain is that the Baatezu and the Tanar'ri have the most extreme hate for each other. The conflict is massive, spanning entire planes, and hosting battles featuring an almost infinite number of fiends. However, in all the time the war has been fought, no side has been able to gain a definite, permanent advantage. Some believe that this can never happen, but it is certain that if one side prevailed over the other, the entire multiverse would be damned. If either race of fiends were to achieve supremacy of all the Lower Planes (with the added question here of how long the Yugoloths would resist if attacked), they would have nowhere else to turn their attention than to the rest of the Planes. It is uncertain whether the forces of the Upper Planes, mighty though they are, would be able to hold them at bay indefinitely.

In fact, in the past some celestials have even supplied weapons to one side or the other. This must not be interpreted as cowardice on the part of the Upper Beings, for it is not in their nature to seek the physical destruction of all things evil, and in many cases they have their own agenda. However, this appears to be a reliable indicator that even the beings of the Upper Planes do not want the war to end.

The war spills over on many different planes, and there are even some that believe that every plane has at one time been touched by it. Many races fight in it besides the two races of fiends. One can even gain money by fighting in it or selling weapons to combatants. It is unclear why someone might choose to do this, since dealing with demons and their like is very tricky, losing your life is a virtual certainty, and after death in this war you’re not very likely to go to a place of happiness and joy. Still, some choose to do it.

Sometimes people wholly unaware of what the war is, mostly first-time visitors from the Prime Material Plane to the other planes, ask to see the war. And surprisingly, sometimes this request is granted. Such a trip is not recommended however, since even the company organizing it offers no guarantees, war can shift paths rapidly, and fiends can reach you quickly.

Tactics
It's interesting to note that of the vast numbers involved in the war, the majority of combatants are fighting for the tanar'ri - the baatezu are able to annihilate much larger groups of the chaotic fiends through their use of tactics, training and formations. The tanar'ri fight in the opposite method, simply sending horde after horde of undisciplined raw power at the enemy.

Beyond the general lack of discipline working against the chaotic side of the war, the entire army has been pressed into service [though true to chaotic nature, there are execptions who volunteered]. The molydeus demons are in charge of sending troops and provisions to the front, and they simply round up and ship off whoever they can find in the abyss, taking any found items they don't want for themselves and sending those on as well.