Earth's Final War was the last great armed conflict fought by humans on their home planet of Earth. It resulted from too much bio-engineering and genetic manipulation of a certain part of the planet's population.
In the late first millenium of the Diaspora of Man, the Slav Supremacists of Ukraine decided to create a superior human. Their genetic laboratories procuded a line of "super soldiers" with enhanced strength, better reflexes, faster healing, and (allegedly) enhanced intelligence. These soldiers were then used to attack other nations in an attempt to gain dominion over the planet.
In reaction to the Ukrainian threat, the Asian Confederacy proceeded to create its own genetically enhanced warriors. These had implanted and natural weaponry on their bodies, and a metabolism that was enhanced to the point that they "burned out" in under twenty T-years. In fact, their combat gear included intravenous concentrated nourishment to keep them alive. Their genetic makeup also left them sterile. They were used primarily as special operations troops, and became megalomanic due to their perceived superiority over normal humans. Eventually, they turned on their creators in the Beijing Coup and took control of the Confederacy and the cloning farms they were created in.
At this point, the Confederacy was in the process of defeating the Ukrainians, but the chaos and political infighting after the coup turned the war in Europe around. The Western European nations had been busy genetically modifying diseases such as anthrax, botulism, bubonic plague, meningitis, typhus, cholera, and the Ebola virus. Having integrated "kill switches" and stockpiled disease-specific vaccines, they believed themselves to be safe and employed the weaponized diseases agains the Asians. Initially, they had almost exactly the desired effect, but after three T-years the pathogens started spreading and mutating among the civilian population, and eventually found their way back to Europe, largely immune to the vaccines their creators expected to protect them.
Together with the weaponized nanotechnology deployed by the powers of the western hemisphere, the diseases managed to almost completely wipe out Earth's human population. They also jumped to the colonies on Earth's moon, Luna, and the neighboring planet Mars. (CS3)
It took almost five centuries for Earth to recover from the carnage. (HH3)
Beowulf led a number of human colonies in sending the Rescue Fleet to the Sol System. This relief effort was able to keep the survivors alive, and eventually led to the creation of the Beowulf Biosciences Code and, indirectly, the Solarian League itself. (CS3)
By the 20th Century PD, the Final War had become a distant memory to most humans; the irony of its name was also not lost on many, as it was in fact the last war fought on Earth, but not the last one fought by humanity by an long shot. Beowulfers remembered it clearer than most, since it had so profoundly an impact on their society and their resolve to end all efforts of creating a "master race". (CS3)
A few descendants of the Ukrainian super soldiers managed to survive well into the 20th Century PD, calling themselves the "Sacred Band" -- others referred to them pejorativly as "Scrags". (HHA3.3: FtH)