Since we don't know the circumstances under which the Amazons arrived on Themyscira at this point in time, this is an unanswerable question.
Yes, they were to guard Ares, but is there a Doom's Doorway?
Were they sent to Themyscira as punishment for disobeying the goddesses when they freed themselves from Hercules and his men?
Those questions have remained unanswered since the New 52.