AVATAR: THE LAST AIRBENDER
A hundred years ago, the Four Nations lived in peace and balance. The balance of the nations was disrupted when Avatar Roku -- the incarnate spirit of the world, and master of the four elements -- perished. Immediately thereafter Firelord Sozin began a war to conquer the Earth Kingdom and, in anticipation of the reincarnation of the Avatar into the Air Nomads, exterminated them summarily.
The last hundred years have been an exercise in misery and hopelessness for the rest of the world. Under the leadership of Firelord Azulon, the Fire Nation exterminated much of the Southern Water Tribe and left those few who remain alone and nearly defenseless. His son, General Iroh, was the only man to breach the great wall of the city of Ba Sing Se -- thought to be impenetrable -- which was only spared occupation when Iroh's son was killed, and the General lost his desire to fight.
The current Firelord -- Ozai, second son of Azulon, and brother of Iroh -- has nonetheless brought the Earth Kingdom to its knees. The city of Omashu, ruled by King Bumi, has long since fallen. The only remaining Earth Kingdom stronghold is Ba Sing Se, where many refugees from all over the world have concentrated themselves in the hopes of escaping the war. Were it to fall, the only bulwark that would remain against the Fire Nation is the Northern Water Tribe, which was saved from destruction at the hands of the late Admiral Zhao by the Avatar and the spirit of the Ocean.
Where the Avatar has since gone, none are sure of. But his return has brought hope to the people of the world, and the impending victory of the Fire Nation might not be as certain as it once was.