Ares, the god of war, had given a belt to Hippolyta, the Queen of the Amazons, and for his ninth labor, Heracles had to obtain the belt and bring it back to King Eurystheus, who had promised it to his daughter. At first, Hippolyta was willing to give the belt to Heracles, but Hera stirred up trouble (no surprise!), which led to a confrontation between Heracles and the Amazon warrior women. Heracles killed Hippolyta in order to obtain the belt, which he brought back to King Eurystheus. There are various versions of the story; you can read more at Wikipedia.