Incursion of barbaric human tribes into elven territory resulting in a bloody war.
The Human population had just barely recovered from the decimation by the demons before the Fall. But there was no order, no honor. The humans' was a crude and martial culture at that time. Humans bred and expanded their domains and were heedless of the sacred borders of the elven realms.
The elves were powerful but the humans were many. A bloody war ensued.
And so the war between the humans and the elves had ended.