Siege of Ba Sing Se

The 600-Day Siege of Ba Sing Se was a massive battle fought between the Fire Nation and the Earth Kingdom six years before the current events in the Avatar World. It is one of the largest battles of the century-long War in recent time.

History
The Siege of Ba Sing Se was started by the Firebender armies being led by the now-retired General Iroh. Iroh was at that point helping his nation to conquer the Earth Kingdom and win the War for his people. Iroh knew that the only way to secure total victory over the Earth Kingdom would be to capture or destroy the country's massive capital, Ba Sing Se, so he launched an assault against the city's famous and supposedly impenetrable Outer Wall, which defended the city from outside forces.

Battle
Iroh spent 600 days laying siege to the Earthbender forces defending the Wall, and during that time, the Earthbenders fought hard to defend their freedom and proved to be worthy adversaries for the Fire Nation forces. However, mere days before the siege ended, Iroh was finally successful in tearing down an enormous section of the Outer Wall, paving the way for an invasion. Iroh's Firebenders flooded into the city's agrarian zone and cleared out the resisting Earthbenders. The Earth Kingdom general defending the Outer Wall surrendered to Iroh, almost assuring victory over the Earth Kingdom. However, Iroh's luck took a turn for the worse. His beloved son, Lu Ten, was killed on the front lines of the battle by Earthbending troops within the last days of the battle. When this news reached Iroh, he was distraught with grief and lost the will to fight the battle any further. Iroh called off the attack and came back home a sad and disgraced man.

Aftermath
The Siege of Ba Sing Se ended in victory for the Earth Kingdom, and the Outer Wall was eventually repaired. Iroh was derided by his people for letting his own emotions interfere with the outcome of the battle. While his nephew Prince Zuko, Princess Ursa, and his father, Fire Lord Azulon, sympathized with Iroh's loss, Princess Azula and Crown Prince Ozai were disgusted by Iroh's decisions. Ozai, who was Iroh's younger brother and heir apparent to the throne, took advantage of Iroh's loss and requested that his father Azulon let him take the throne in Iroh's place, as Lu Ten's death would have left Iroh without a proper heir. Azulon was infuriated by this proposal and swore to punish Ozai. However, before this could be done, Azulon died suddenly and Ursa disappeared. Iroh was too upset to fight for his throne, so Ozai became Fire Lord on Azulon's dying wish, or so it's claimed. Iroh lost his birthright and his son as a result of the siege, and Ba Sing Se would not fall until Zuko and Azula overthrew the Earth King six years later.