The Frontier Incident

Senki Zesshou Symphogear G's Main Event.

This is the name given to the series of events that occurred as a result of the Lunar Attack. Due to the threat of the moon crashing into the Earth, a group sought to ensure humanity's survival by awakening a prehistoric ruin that slept deep within the Japan Sea - known as "Frontier."

Due to complex ulterior motives of both the world's countries and certain individuals, it's safe to say that the full story will never become clear.