Some quick information that will be expanded on in a further post;
The war began 50~ years ago (I believe the exact date is 52).
The Air Avatar did not survive, and was replaced with Ziah of the Northern Water Tribe.
The War in the Earth Kingdom is fairly touch and go for the many Earth Kings for the early years of the war.
Roughly 30~ years ago Omashu is occupied by the Fire Nation
Avatar Ziah arrives and begins turning the tides.
Roughly 20~ Years Ago Ziah liberates Omashu and pushes the Fire Nation back.
16 years ago, the North Pole is attacked and Ziah leads her people to the South Pole for her people to flee.
Ziah is killed while helping her people escape by Earthbending allies.
Schisms form as rumors of Ba Sing Se's treachery reaches Omashu and other liberated provinces.
Rumors spread by Ba Sing Se for years is that the Avatar is gone, until they announce the arrival of a new avatar 8 years ago.
That's essentially the picture of it all. The Fire Nation pretty much took over the North Pole, which apart from some isolated villages or refugees from the war, is depopulated. The South Pole is bolstered and facing some cultural issues as the more patriarchal society of the North clashes with South Pole ideals. The Earth Kingdom is a mess of disunity, with the only reason why Omashu and Ba Sing Se aren't fighting one another is because of the presence of the Fire Spitters.