Katara: Aang......I think you were in that ice for almost a hundred years. The Firebenders started a war. Aang: I left a few days ago. Katara: Aang, the Fire Nation knew the Avatar would be born into the Air Nomads... so they exterminated all the Air Nomads.
Katara tells Aang the story of how all the monks were exterminated by the Fire Nation.