The Gates of War are, in the Aeneid, both a metaphor and a real, physical set of doors in Lavinium. (The Rome of Virgil's time also had physical Gates of War, which they closed, with much festivity, at the end of wars. Augustus famously closed the gates three times in his reign, more than they had ever been shut in Roman history.) The Gates are part of the temple of Janus, a two-faced god of beginnings and endings. When the war of the Trojans against the Latins begins, Latinus doesn't want to open the gates of war in Lavinium, so Juno does it for him. The Gates connect Rome's mythological past to the time of Augustus, and graphically demonstrate the extent of Juno's meddling.
The Gates of War Symbol Timeline in The Aeneid
The timeline below shows where the symbol The Gates of War appears in The Aeneid. The colored dots and icons indicate which themes are associated with that appearance.
...from the mountains and ask Latinus to declare war. Latinus is reluctant to open the gates of war , which would serve as the declaration. Juno intervenes, opening the gates of war herself.... (full context)