Ares was an Old God who was commonly worshiped as the "god of war," and the most fearsome son of Zeus, who was worshiped by the Ancient Greeks and Romans (though the latter civilization would give him the name Mars).
Millenia ago, Zeus created the Amazons, as a fail-safe against Ares's power. Ares would eventually instigate the War of the Gods, successfully killing almost all other Old Gods and wounding Zeus, but was ultimately defeated and grievously wounded by the King of the Gods and was unable to stop his father from fathering Wonder Woman and creating Themyscira, leaving him temporarily incapacitated and permanently much weaker but still succeeding in becoming unopposed in his quest to destroy humanity as Zeus died from the severe wounds Ares inflicted on him after creating Themyscira.
Ares spent many years influencing humanity to create weapons which he hoped they would use to destroy each other. Ares would then reappear during World War I, taking the disguise of Sir Patrick Morgan, and inspiring Dr. Poison to develop a powerful and deadly gaseous weapon, which desired to use to destroy humanity, a race which he had developed bitter jealousy for. In the end, however, Ares' plans were thwarted, with Wonder Woman killing him and ending World War I and his violent influence on humanity for good.