Hi To Your Loved Ones.
The events one hundred years before the beginning of the show are revealed gradually and out of order throughout the series.
More than a century before the beginning of the series, the ruler of the Fire Nation, Fire Lord Sozin, planned a world war to expand his territory; however, the Avatar at the time, Roku, prevented him from doing so. He waited for the Avatar's death, whereupon the latter would be reincarnated as an Airbender named Aang. Aang's Avatar status was revealed to him while still a child, despite the protests of his mentor Monk Gyatso. Aang, fearful of his new responsibilities and of separation from Gyatso, fled his home on his flying bison, Appa. The two were subsequently forced into the ocean by a storm, and Aang's protective Avatar State encased them both in an iceberg, in suspended animation. Fire Lord Sozin, in an attempt to kill the new Avatar, carried out a genocide of the Air Nomads, leaving twelve-year old Aang as the eponymous "last Airbender".
The war continued for one hundred years. Sozin was succeeded as Fire Lord by his son Azulon, and Azulon by the younger of his two sons, Ozai, the current ruling Fire Lord at the time of the series. The older son is Iroh, who laid siege to the Earth Kingdom capital of Ba Sing Se for over 600 days when he was younger.