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 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 because it was a real one-trick plot hook, a lot because it really made the cosmic forces of good seem like useless pushovers incapable of fighting evil themselves, and many 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 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.