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The War of the Gods was an ancient conflict of godly proportions that raged between Ares and all other Olympian Gods, due to none of them supporting his hatred of humanity, his bloodthrist, or his extremist views. Ares would ultimately prevail, leaving him free to continue his violent corruption of humanity, at least until the "Godkiller" Diana resurfaced.
Zeus ruled over the Olympian Gods as the most powerful one of them, and eventually, millennia ago, fathered Ares, among others. At some point afterwards, Zeus proceeded to create humanity, making them in the gods' likeness, and imbuing them with many virtues.
Ares, however, developed jealousy and hatred for the new race, and promptly began to corrupt their virtuous nature by instigating wars among them. Zeus, determined to protect humanity, created a new race - the Amazons, whom he imbued with the ability to spread love, compassion, and a mutual understanding among all human nations. This, however, would only induce Ares to violently rebel against Zeus and all other gods for not sharing his hatred, bloodthirst, and extremism.
Growing more powerful from the violence of the godly war he had instigated, Ares fought and slayed all of the other gods as they attempted to bring him down, until only Zeus was left. Before Zeus' battle with the God of War, he conceived a child with Hippolyta, Queen of the Amazons, in case he lost his life fighting Ares. Zeus then fearlessly confronted his malevolent son in combat, won and drove him off, but was mortally wounded himself. Zeus's foresight payed off, as he fathered Diana (as a contingency "Godkiller"), and before succumbing to his wounds, he created Themyscira, an island to keep the Amazons, and more importantly, his daughter safe from Ares, until Diana's time came to become the "Godkiller".
As the only remaining Olympian God after the war's conclusion, Ares was now free to continue his violent corruption of humanity for over 5 millennia, all the while patiently waiting for his only remaining threat, the "Godkiller", to reveal herself.