Coup of Ba Sing Se

This battle began when Princess Azula of the Fire Nation, who had infiltrated the city of Ba Sing Se, arrived in the crystal catacombs of old Ba Sing Se, and fought against Team Avatar. Azula had turned the Dai Li, the cultural protectors of the city's heritage, against their leader, Long Feng.

Background
The banished Prince Zuko and Katara had been thrown into a part of the catacombs as prisoners earlier on by the Dai Li. When Avatar Aang and the retired general Iroh arrived in the catacombs, they found the two captives, but after Aang and Katara left to find their friends, Zuko and Iroh were confronted by Azula and her Dai Li agents. She pleaded with Zuko to join sides with the Fire Nation again, while Iroh insisted he mustn't listen to her. As Aang and Katara flee the catacombs, they are stopped by Azula, and then Zuko. In a silent stand-off, Zuko's side becomes apparent to everyone as he shoots a fire blast at Aang.