From the introduction:
One thousand years ago there existed a man known as the Sage of Six Paths who performed feats which elevated him to the status of legends and gods within the minds of those scattered few who still know of his existence.
He defeated a ten tailed monster that threatened to consume the world, before travelling near and far to teach man how to make use of chakra. It was his hope that this power would allow people to push past their hatred, find mutual kinship, and end all conflict.
Humanity instead - in part because of dark conspirators unknown to history - went on to wage near constant bloody warfare ever since. War led to death, and loss, and hatred, creating a self-perpetuating cycle of vengeance and wrath. This period is referred to as the Clan Wars.