Calamity War (Another Poetic Spartan)

The Calamity War, originally referred to as the Long War, was a devastating civil war fought between the Earth Sphere and the Outer Sphere sometime before the establishment of the calendar. Lasting for several decades, it led to the creation of a third faction: the Mobile Armors.

It was widely regarded as the largest and bloodiest conflict in Human history, having resulted in the deaths of about a quarter of the overall Human population.

Consequences and Legacy
The calendar was officially established by  under their Vingolf Proclamation.