About the Four Nations

For thousands of years, the world of avatar has been governed by the Four Nations. These are the Water Tribes, Earth Kingdom, Fire Nation and Air Nomads.

Each Nation occupies a certain amount of land. They are supposed to be content with their land, and not invade other kingdoms. That's where the Avatar comes in. If one Nation occupies another, the Avatar is supposed to stop the fighting. Kind of like the UN in the real world.

The reason that the Fire Nation managed to wipe out the Air Nomads, and take over some part of the Earth Kingdom, is because at that time, there was no Avatar to help them. After Sozin killed Roku, Aang was just a child. Then, at just twelve years old Aang ran away from the Southern Air Temple and froze himself in a block of ice for one hundred years. While he was frozen, the Fire Nation did all this damage.