The Battle of Ba Sing Se, also known as the Liberation of Ba Sing Se was the one of the final battles which occurred in The War. The secretive Order of the White Lotus, led by Iroh, penetrated into Ba Sing Se and fought with occupying Fire Nation forces in Ba Sing Se in order to free the city in the name of the Earth Kingdom.
The large Fire Nation garrison outnumbered the small Order, and the arrival of Sozin's Comet increased their power. However, the Order consisted of the most powerful benders and fighters in the world, including some firebenders of their own. This meant that the Order had a considerable tactical advantage, leading to their victory.
The battle was a decisive defeat for the Fire Nation armies, who were also defeated at the Battle at Wulong Forest.
The city of Ba Sing Se was the last Earth Kingdom stronghold the country had available to sustain its war effort against the Fire Nation after the fortress city of Omashu fell to the enemy several months prior. Throughout the entire course of the War, Ba Sing Se was the country's major source of soldiers, food, equipment, etc. Ba Sing Se was once considered unconquerable due to the Fire Nation's failure to penetrate its walls.
However, roughly three months prior to the arrival of Sozin's Comet, the Earth Kingdom suffered a heavy defeat when Princess Azula and her two allies, Mai and Ty Lee, infiltrated the city disguised as Kyoshi Warriors. Through her expert manipulation, Azula was able to secure control over the Dai Li, the power-hungry cultural enforcers of Ba Sing Se. She was also able to enlist the help of her exiled brother Prince Zuko, who was desperate to return to the Fire Nation. Together, they overthrew Earth King Kuei and defeated Avatar Aang in a coup d'etat that left Azula in total control of the city, denying the country the resources necessary to win the War. Within days, the Dai Li demolished the city walls and the Fire Nation Army conquered the city, securing almost total victory over the Earth Kingdom.
The people of the world realized that if balance was to be restored to the world, Ba Sing Se had to be restored to the Earth King's rule. Earthbenders organized underground rebellions to resist the occupation, which served to deny the Fire Nation full control of the city. However, their efforts would not be enough to drive out the invaders before the Comet arrived and the city became threatened by Ozai's airship armada.
In an effort to save the city from destruction, the Order of the White Lotus, a secret organization transcending world boundaries, initiated a mission to liberate the city on the day of the Comet's arrival. With its power, the Order's Firebenders would give the Society's small army a significant advantage over the Fire Nation Army.
Prior to the arrival of Sozin's Comet, ex-General Iroh, a Grand Lotus of the Order, sent out a signal to other members of the Society advising them to gather at Ba Sing Se. The day before the scheduled operation, the Order encountered Prince Zuko and his allies. Iroh and Zuko were reunited and it became clear that if the war was to end the Fire Nation would need to be decisively defeated at Ba Sing Se.
Upon the Comet's arrival, the Order laid siege to the city. With a single powerful blast, Iroh breached the Inner Wall. The Order proceeded to advance towards the wall to enter the city. Dozens of Firebenders from atop the walls rained fire down at the invading army with no effect, and the Order broke into Ba Sing Se. Heavy fighting took place at the main plaza of the Outer Ring and the streets became littered with broken down shells of the occupiers' tanks. The Order advanced to the center of Ba Sing Se as the retreating army was encircled in hundreds of pockets throughout the city.
Hours of intense combat passed, but the outcome became clear. Iroh is seen burning the Fire Nation flag that hung over the Earth Kingdom Royal Palace, symbolizing that the Earth Kingdom was free once again with the Fire Nation's defeat.
Little is seen of Ba Sing Se after the end of the War, although a few kids are seen playing Earth Soccer, using stacked up Tundra Tanks (the ones Bumi defeated during the battle) in the game, and Iroh, along with Team Avatar is seen at his tea shop, the Jasmine Dragon. Much of the city was damaged in the battle, and the Walls needed extensive repairs. However, the city was freed from the Fire Nation's rule and its citizens enjoyed a newfound sense of freedom. With the appointment of Fire Lord Zuko, the Fire Nation rebuilt relations with the Earth Kingdom. It is likely that the Dai Li have been stripped of their authority over the city and the suspension of free speech lifted.