Janus is one of the oldest Roman gods. He was represented as having two faces: one look forwards, and the other – backwards. Janus helped the Romans to defend their city, that is why in time of war the door of the Temple of Janus was always kept open, so that the god could come to the aid of Romans. The door was only closed if the Roman Empire was at peace.
See: Peter Paul Rubens Temple of Janus.