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 bloodthirst, or his extremist views. Although Ares managed to kill all the other Old Gods, in his one-on-one fight with Zeus, the King of the Old Gods ultimately prevailed, but died due to the severe injuries he sustained and the strain needed to create Themyscira, leaving Ares free to continue his crusade to destroy humanity until the "Godkiller" Diana resurfaced.
Zeus ruled over the Olympian Gods as the most powerful of them, and eventually, millennia ago, fathered Ares, among others Gods and demigods. At some point, Zeus presumably proceeded to create humanity, making them in the gods' likeness, and imbuing them with many virtues.
Ares, however, slowly begun to develop a jealousy and hatred for this new race, and was more than determined to show his father how weak and unworthy humanity really was by influencing their virtuous nature and instigating war and conflict among them. Zeus and the Olympians wanting to preserve humanity, created a new race - the Amazons, whom they 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 wicked point of view.
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. The King of the Gods then fearlessly confronted his malevolent son in combat, winning and casting him out from Olympus. Knowing that one day Ares would try to find a way to achieve his goal, Zeus conceived a child with Hippolyta, Queen of the Amazons. Zeus's foresight payed off, as he fathered Diana (as a contingency "Godkiller"), and using the last of his divine powers, 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 weaker but free to continue his violent corruption of humanity and regain his powers for over 5 millennia, all the while patiently waiting for his only remaining threat, the "Godkiller", to reveal herself.