The Avatar cycle is broken before the war, the war rages for 100 years and finally ends with the fall of ba sen sei. The only real threat left to the fire nation are the water benders, the most illussive and arguably the most powerful benders. To restrict this risk, the fire nation kills every male bender in sight and capturing healers and children who aren't trained in combat. Healers sent to hospitals, boys to labour camps and the girls into the entertainment industry. Katara age 12, is captured and begins her life as a high class water dancer.
(Zutara fanfic, anng is not the avatar or in the story)