Air Nomad Genocide

The Air Nomad Genocide was an enormous massacre committed by the Fire Nation that resulted in the complete extinction of the Air Nomads. The only survivors were Aang, Momo and Appa. The effects of this cultural destruction empowered the Earth Kingdom and the Water Tribe to arm themselves and declare war on the Fire Nation, leading to the large-scale war that continued for 100 years.

Preface
Days before Aang's birth, Roku was fighting a volcano that erupted on Roku's Island, and Sozin came to his aid. Towards the end of the battle, the volcano re-erupted and Roku breathed in the toxic fumes. He beckoned Sozin to help him, but Sozin realizes that his plans would be possible without his former friend's interference. Sozin proceeded to leave Roku for dead in order to carry out his desire for world conquest. Roku died there along with his dragon, Fang. Shortly after Roku's death Aang was born.

12 years later, Aang became a master Airbender and the youngest Airbending master in history. Because of the threat of oncoming war, the monks, including Monk Gyatso, told Aang of his identity as the Avatar, when in normal circumstances the Avatar is told of his or her identity at the age of 16. Gyatso attempts to maintain Aang's carefree lifestyle to minimize this new and colossal burden placed onto him, but the monks decide to split the two up and send Aang to the Eastern Air Temple in order to continue his training. Little did Gyatso and the other monks know that Aang had eavesdropped on their conversation from a secret hiding place. Unsure of what to do, he flees on Appa towards the south, but gets caught in a fierce storm. He and Appa fall into the ocean and begin to sink which forces the Avatar State to intervene. Utilizing both Airbending and Waterbending, the state-induced Aang froze himself and Appa presumably with the thought that the iceberg would be carried to a warm location to thaw later on. But the storm carried his frozen body south towards the south pole, and therefore it never thawed.

Plot Behind the Assault
Sozin knew that Roku's successor would be born an Air Nomad, and he plotted out the possible assault against them, possibly with the same cunning he used to win the Battle of Han Tui. He then waited for the comet (which would later be renamed Sozin's Comet) which arrived once every 100 years, in order to execute his plan. When it arrived, it gave Firebenders the power needed to attack the Air Nomads.

The stage was set for the largest, bloodiest, and possibly the only genocide that occurred during The War.

The Genocide
Due to its proximity to the Fire Nation and assuming they attacked the temples in a west-to-east course, the Western Air Temple was attacked first, and was of strategic importance due to its under-the-cliff-side location. This first offense was so well executed that this temple was the most undamaged of the four air temples (until Combustion Man partially damaged the fountain structure in the Western Air Temple).

Next, they headed southeast, towards the Southern Air Temple. This battle was quite possibly the toughest fight of the conquest, since the Airbending monks, including Monk Gyatso, where highly trained and very powerful. As a result, the Fire Nation experienced many casualties, despite them winning the fight. The main cause of the casualties was Monk Gyatso's superior fighting skills. It is suggested he fought nearly a hundred soldiers without rest. He was likely knocked to the ground by a fire blast and because he was fatigued from his efforts. He may have fought several more soldiers in this position before he finally expired (which is why Aang finds his corpse positioned as such). This temple was the only temple in the series shown with Fire Nation corpses lying on the ground near the foot of the temple. This is likely because the army was greatly weakened from the battle and lacked enough manpower to clear the bodies.

Before travelling north, towards the Northern Air Temple, they likely regrouped and received reinforcements. At that temple, it likely was still a tough fight, but the fresh soldiers once again proved superior against the Airbenders there. Lastly, they headed southeast once again, this time to the Eastern Air Temple, where they won a swift victory and wiped out the Sky Bisons as well (with the exception of Appa). Once they killed all the temple's inhabitants, they set fire to the three pagodas of the temple as a sign of supremacy, which is why this temple is likely the most heavily-damaged of the four temples. Years later, Guru Pathik arrived at the temple, and restored most of it.

It is equally possible that the Fire Nation mounted roughly simultaneous assaults on all the temples. While more difficult, and thus less likely, then a sequential attack this would have made it impossible for any temple to send a warning to the others before the others fell under attack, and would have made mounting a unified defence (at any temple) all but impossible.

Immediate Aftermath
The only Airbender to survive the onslaught was ironically the one that the Fire Nation sought to kill in its quest for world supremacy; The Avatar, Aang. By fleeing the temple just days before the first attacks against the Air Nomads, he saved himself. However, Aang felt guilty about fleeing and believed he could have defeated the invading Fire Nation forces and saved the Air Nomads which he revealed to Katara during "The Storm".

It is currently unknown if any other Airbenders survived for sure, but there is so far no indication of other survivors. The loss of the Air Nomad civilization also puts the continuation of the Avatar Cycle in question. If there are no other Airbenders living, then it is possible that the Cycle is broken and will not continue with the next Air Nomad.

It s rumored that the power of the comet revitalized Sozin's life force and allowed him to live another 20 years. Sozin died at the old age of 102. He spent the last 20 years of his life looking for Aang, who by that time had been frozen in an iceberg beneath the ocean for several years. Sozin died before he could find Aang but he spawned a legacy and dynasty that lasted for 100 years.

General Aftermath
100 years after the genocide, in "The Boy in the Iceberg", Katara and Sokka freed Aang and Appa from their frozen state. Aang soon finds out of the battles with the Air Nomads. In "The Southern Air Temple", Aang visits his former home, where he discovers countless Fire Nation corpses along with Gyatso's frail corpse. Overcome with emotion, he enters the Avatar State and encases himself in a sphere of air which threatens to destroy the surrounding structures, but Katara calms him down. Inadvertently, his transformation into the state was inside the temple and caused the statues and tapestries of every other Avatar within every Avatar temple in the other nations to have white, glowing eyes, alerting the world that the Avatar has returned.

In "The Northern Air Temple", Aang and the gang arrive at this temple and discover it is inhabited by Earth Kingdom refugees who were forced from their homes by a flood years before. Aang is angry to see the temple vandalized by The Mechanist's modifications. However, he stated that just like the hermit crab he saw earlier, that they found a new "shell" to call home and allowed them to stay.

In "Appa's Lost Days", Appa returned to the Eastern Air Temple and encountered Guru Pathik there. At first, Appa was hostile towards Guru Pathik. However, Appa eventually warmed up to Guru Pathik who told him Aang's location and gave him a message to give to Aang. Upon his arrival at Ba Sing Se, Appa was captured by Long Feng and held captive by the Dai Li at Lake Laogai.

In "The Guru", Aang goes to the Eastern Air Temple to unlock his seven chakras in order to gain control of the Avatar State but he left after trying to unlock his last one, blocking the chakra. Aang was then struck by a lightning bolt from Azula while attempting to unlock the seventh chakra, severing his connection from the Avatar State. Katara managed to revive him by using the spirit water given to her by Pakku in "The Avatar State". In the Escape from the Spirit World online interactive game, he restores his connection to the Avatar State, but has to unlock all his chakras again in order to use the Avatar State once again (though he could still meditate).

After the failed invasion, the gang took refuge in the Western Air Temple. When Combustion Man showed up to fight he started to destroy parts of the temple. But when Sokka struck him in the head with his boomerang, Combustion Man blew himself up. Later Azula and several Fire Nation airships came and started throwing bombs at them, thus destroying parts of the temple. Aang and his friends were forced to leave the Western Air Temple after that. The current state of the Western Air Temple is unknown.