The six hundred-day Siege of Ba Sing Se was an assault by the Fire Nation on the Earth Kingdom capital six years before the end of the Hundred Year War. It was one of the largest and most legendary battles of the latter stages of the century-long war.
The Siege of Ba Sing Se was an attempt to expand Fire Nation territories further into the Earth Kingdom, started by firebender armies being led by the now-retired GeneralIroh. His hope was that, by conquering the mightiest city in the Earth Kingdom, he could finally secure victory for the Fire Nation.
Around 94 ASC, General Iroh and his army besieged the Earth Kingdom capital of Ba Sing Se. The siege was initiated after Fire Nation infantry began attacking the Outer Wall of the city, hoping to breach the walls in order to secure a path for an invasion. The defending earthbenders of Ba Sing Se fought back, managing to hold them off for a long period of time. The turning point of the siege came when General Iroh finally breached the Outer Wall. After a long period of battle, the Fire Nation secured the foothold that they desired. The breach in the wall paved the way for invasion, and after Iroh's victory at the Outer Wall, the general defending it surrendered.
The Fire Nation advanced into the Agrarian Zone and continued their onslaught. However, General Iroh's morale was crushed upon receiving news that his son, Lu Ten, had been killed in battle. Heartbroken and demoralized, Iroh lost his will to fight, and realizing that he had lost many troops and that their morale was wavering, he withdrew from the battle, thus ending the six hundred-day siege. The attempted invasion failed, though the Fire Nation unsuccessfully attempted to recapture the city in many future assaults.
The given information suggests that the Fire Nation mustered and made use of many of their available resources, and that the Fire Nation used a few divisions according to the maps shown in the past as well as the pile of scrolls near Iroh's office.
The Siege of Ba Sing Se resulted in victory for the Earth Kingdom, and the Outer Wall was eventually repaired. Despite this victory, the long siege left many bitter feelings toward the Fire Nation and Iroh, specifically. These feelings were displayed when an earthbender captain captured Iroh, longing for vengeance for the chaos and violence he and his nation had wrought during the siege. Iroh was derided by his people for letting his own emotions interfere with the outcome of the battle. However, many in the Fire Nation who served under his command, despite the ultimate failure of Iroh's siege and even after his overt treachery in Ba Sing Se, praised him as "a great leader and a great man". While his nephew PrinceZuko, sister-in-law Princess Ursa, and father, Fire LordAzulon, sympathized with Iroh's loss, his niece Princess Azula and younger brother Prince Ozai were disgusted by Iroh's decisions. Ozai, intending to take advantage of Iroh's losses and the failure of the siege by requesting that his father Azulon let him take the throne in Iroh's place, claimed that 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, emotionally wrecked, was too upset to fight for his throne, and therefore Ozai became Fire Lord on Azulon's alleged dying wish. Iroh lost his birthright and his son as a result of the siege, and Ba Sing Se did not fall until Zuko, Azula, Mai, and Ty Lee's recently acquired Dai Li agents overthrew the Earth King six years later.
In The Last Airbender, the Siege of Ba Sing Se was shortened to only one hundred days instead of six hundred.
Under Iroh's command, the Ba Sing Se wall has been destroyed twice: the first time was during this Siege, when he intended to conquer the city in name of the Fire Nation, and the second time was during the passing of Sozin's Comet in 100 ASC, when he aimed to liberate the city from the Fire Nation.