History

In the year 0 AG (After Genocide) the Sozin Comet passed nearby Earth and empowered the Fire Nation to commit genocide against the Air Nomads and launch a full-on invasion against the Earth Kingdom and the Water Tribes.
Thirty years after, in 30 AG, the Four Nation games begins, in a world consumed for an ongoing war that has already trascended beyond a generation.
You can consult the entire history of the Four Nations game in The Timeline