Book 3 Chapter 10 "THE DAY OF BLACK SUN, PART: 1: THE INVASION"

On the day of the solar eclipse, many allies featured in previous episodes, such as Haru and his father, the swampbenders, Pipsqueak and The Duke, and The Mechanist and his son Teo, reunite with Team Avatar to launch the invasion of the Fire Nation. Aang kisses Katara as a sign of his strong feelings for her, worried that he may not return. Meanwhile, Zuko leaves a letter to Mai on her bed, and tells a portrait of his mother that he is finally going to set things right. He grabs his broad swords and a bag and heads out the door. The invasion force successfully circumvent several layers of Fire Nation defenses and reach the capital shores, where they begin a grueling assault towards the palace under constant onslaught from Fire Nation forces. Sokka and Katara's father Hakoda is injured in battle, forcing Sokka to assume command of their forces. Aang reaches the Fire Lord's palace, only to find both it and the entire capital city abandoned.