The entire span of the history of mankind is but a heartbeat when compared to eternity. It is smaller still when compared to the might Blood War that is fought between the baatezu and tanar'ri. For as long as there has been time, these mighty races have clashed with each other in a war of ultimate genocide and wholesale destruction. There can be no compromise in this war. Only total destruction of the enemy will end the Blood War. Such has it been since the creation of time.
Necromancers and sages have diligently studied the Blood War as a favored topic among such circles. Despite their efforts, it is difficult to learn of something that began so long ago - the origins of the Blood War are so remote as to be incomprehensible to even the most wise sages of the Prime Material plane.
The sages of Toril perhaps sum up the Blood War best. They tell of The One Who Remains Hidden who created the universe. Desiring perfect balance, he created three races of beings: those of good, those of neutrality, and those of evil. Of course, these original races subdivided still further into lesser races - splinters of evil were spread thinly among lesser evil races, but the greatest block of evil was split evenly between the tanar'ri and baatezu. Each possessed but half of a perfect sphere of evil, and each coveted the other's half above all things. The Blood War rages over the sphere of evil that both sides desire, but neither side can completely capture.
Actually, the Blood War may be regarded as the salvation of goodness. Were the two greatest teams of evil to quit their eternal struggle, joining forces in a wanton spread of their corruption, the champions of goodness would be hard pressed to stop them.
Whatever its origins, mankind plays but a small role in the great war. As far as either the baatezu or the tanar'ri are concerned, mankind merely exists to be dominated in order to provide a foothold in the endless war. It is possible that the Prime Material plane itself holds some sort of energy that would be valuable in the waging of the Blood War - mankind may be merely the innocent bystander in the efforts of the fiends.
Concerning the Blood War, Mordenkainen writes:
I prepared all as had been described in the ancient tomes. Every detail was checked a dozen times to insure it was correct, and then it was checked a dozen more times. When all was complete, I performed the act that caused me to pass into a deep sleep in which I dreamed dreams of both history and prophesy. I learned of many things during that ritual, but of one thing did I learn much: The Blood War.
It was shown to me that the great evil of the two pits of darkness were engaged in a battle of infinity. They clashed together in endless energy and struggle and would likely continue to clash for ever and ever.
When I had seen this battle I shuddered and a great fear came over me. I was inclined to break the trance and leave my dream state. But then I viewed a frightening sight. A thousand times a thousand strands of unknown substance extended from infinity to infinity. They were coils of time and written upon them were the histories of all deeds and places. They passed through all things in their way and tied all things together. And when I saw that the strand that spoke of the Blood War passed through the lower planes and then directly through the heart of mankind, I truly knew fear.