A brief history:
In light of the anti-bending movement going global, the government seized Avatar Korra, and seeing as they could not kill her and just bring forth another Avatar, instead hid her away in an isolated prison as a means to put a stop to her. Troops have been sent to every town to find and enslave benders. Korra, seeing that she was where no one would ever find her, and where her Avatar powers were mysteriously rendered useless, killed herself in a heroic attempt to bring forth the next Avatar, so that he or she might bring peace to a land she could no longer save.
Years pass, and the Earth Kingdom becomes a focus point for government troops, but with bending banned in all nations, the indentity of the next Avatar remains a mystery…