The War spanned centuries, if not millenia. Even during the height of The War no remembered what started the conflict, it merely continued with innumerable grudges on many fronts. Many factions rose, allied, betrayed, and fell. No one knows when or how it ultimately ended, other than at some point in centuries past the fighting ceased, at least on a global scale, and the world was in ruins.
In the intervening years it has been widely accepted that magic was mostly to blame for the destruction of the world and that the Gods turned their back on humanity and left them in despair.