Blood War

The Blood War is an archaic piece of Dungeons & Dragons lore that originated from the Planescape setting. It is, as its name suggests, a war - one mighty enough to make Khorne drool in envy. Fought between the Tanar'ri and the Baatezu, otherwise known as "demons" and "devils," the Blood War traditionally spans the entire multiverse, ensnaring everything directly or indirectly in its path.

In AD&D

In 2nd edition, the Blood War cast a massive shadow over the Planescape setting, and thus over the greater D&D metacontinuity as a whole. The Tanari'ri and the Baatezu, two infinite armies of evil outsiders, wage a bloody war of mutual destruction over the definition of Evil, with demons firmly believing that evil is pointless if it doesn't arise from a place of passion and spontaneity, and devils arguing that evil is pointless if it doesn't constantly keep long-term plans in mind.

The Blood War covers the entirety of the Lower Planes to one extent or another, five infinitely vast planes devoted to pure evil, and untold millions of Prime Material worlds. It is the source of both fiendish races' interest in corrupting and damning mortals, to have an infinite supply of resources and cannon fodder to continue to fight the war with. An entire third race of fiends, the Yugoloths, exist only to be mercenaries in the Blood War. All the denizens of the Upper Planes secretly feed and support the Blood War, bartering weapons and services to whichever side is currently losing, because they know that if the war wasn't keeping the fiends at each others' throats, they would inevitably fall before the onslaught of pure evil.

...Yeah, you can tell that this was written during the 80s and D&D's efforts to be "punk," "mature," and "gritty," can't you?

The Blood War has always been a contentious aspect of the setting. Though support for it has only increased as the editions have rolled on and pushed Planescape further and further out of the spotlight, and fans have adopted thicker and thicker nostalgia goggles to cope (to the point that "I love the Blood War" is practically the defining trait of a "true" Planescape fan these days), there have always been individuals who disliked it. Some thought it was ridiculously oversold as a setting element, some thought it was a real one-trick plot hook (alright, many different hooks, but still the same "influence the War in your little corner of the Planes" story), a lot disliked the emphasis on the Blood War because it really made the cosmic forces of good seem like useless pushovers incapable of fighting evil themselves, and many hated it because it tended to take over the story whenever it got introduced. Planescape: Torment, anyone?

In 3.x

Because Planescape wasn't pushed so hard as a setting here, the Blood War became less important. It was still there, still canonical, but a lot less emphasis got placed on it.

In 4e

4e did a lot of cosmological changes, and the Blood War was one of them. Whilst the Planescape grognards found it especially galling, most fans actually liked it better.

In 4e, the Blood War is sheered of its Alignment-fueled rationale. Instead, the Blood War is fought for a simpler reason. Demons want to destroy everything. Devils want to conquer everything. Naturally, the two goals are incompatible. Adding fuel to the Blood War is the fact that Asmodeus became the God of Sin through stealing a shard of the Heart of the Abyss, the force that created all demons. Naturally, they're kind of pissed off about this, while the devils want to steal the rest of it and become even more powerful.

More dramatically, the Blood War in 4e officially goes through a cycle of hot and cold war; the demons and devils fight reality-shaking battles in which trillions of outsiders and souls are utterly destroyed over centuries of endless war, then the fury dies down to more manageable levels as they rebuild their home planes and their ranks before starting it up again. This probably wouldn't piss off as many people as it does were it not for the fact that the books are officially written under the presumption that it's currently in one of its cold phases, though DMs are given full reign to have it go hot again in their campaign.

In Pathfinder

Perhaps because the Blood War isn't OGL-covered, Pathfinder makes no use of it. Demons and devils fight, yeah, but that's because they're evil, destructive bastards. There's no big philosophical fight between the two. Indeed, as demons in Pathfinder are created from mortal sins, and devils live to corrupt mortals by committing sins, one can say that devils literally create demons.

That said, there are two big inter-fiend conflicts in Pathfinder.

Firstly, the Abyss was originally home to a fiendish race called the Qlippoth, who are very peeved about how mortal sins caused demons to arise. To this end, they want to annihilate all demons and all mortals, to stop any new demons from being born.

Secondly, Daemons - Pathfinder's equivalent of Yugoloths, the "Neutral Evil fiends" - are characterised as omnicidal maniacs. They want to annihilate everything. As both demons and devils have no interest in wiping out the multiverse, they don't get on with daemons. At all.