Air Nomad Genocide

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 are suddenly possible without his former friend's interference. Sozin proceeded to leave Roku for dead, since he despised Roku always attempting to prevent him from world conquest. Aang was then born soon after Roku was killed by a pyroclastic flow from the eruption.

12 years later, Aang is a master airbender, 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/her identity at 16 years of age. Gyatso attempts to maintain Aang's carefree lifestyle to minimize this new and colossal burden placed onto him, but eventually the monks decide to split the two up and send Aang to the Eastern Air Temple (where he first met Appa) in order to continue his training. Little did Gyatso and the other monks know that Aang eavesdropped on the conversation from a secret hiding place. Unsure of what to do, he flees on Appa towards the south, getting caught in a fierce storm that forces the avatar state to intervene, and thus, utilizing both Airbending and Waterbending, the state-induced Aang froze himself and Appa 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 never being 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 (soon named 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.

Now, the stage was set for the largest, bloodiest, and possibly the only genocide occurring in 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 (episode)). 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. He must have fought nearly a hundred soldiers without rest. He was likely knocked to the ground by a fire blast and because he was weakened 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 (including Monk Gyatso's) lying on the ground near the foot of the temple because the army was probably greatly weakened from the battle, not having enough manpower to clear the bodies. Before traveling north, towards the Northern Air Temple, they may have 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. That being done, 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 4 temples. Guru Pathik was possibly in residence within or around this temple before the genocide. He likely went into hiding and only came back after the troops had left.

Immediate Aftermath
The only airbender to survive the onslaught was ironically the one that the fire nation wanted to kill in its quest for complete supremacy, Aang. By fleeing the temple just days before the first attacks on the air temples occurred, he saved himself at the huge cost of his fellow air nomads.

The power of the comet revitalized Sozin's life force to the point that he lived a further 20 years, to die at the old age of 102. He spent those last 20 years looking for Aang, who by that time had been frozen in the iceberg beneath the waves for a long time. Sozin died before he could find Aang, but he spawned a legacy and a dynasty that still continues to the present-day.

General Aftermath
100 years after the genocide, in The Boy in the Iceberg, Katara and Sokka freed Aang from suspended animation by being frozen (and Appa as well), when he 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 around Gyatso's frail corpse. Hit by a wave of extreme sadness, he enters the avatar state and threatens to blow this portion of the temple off its foundations, but Katara calmed him down. Inadvertedly, his transformation into the state was inside the temple, causing the statues and tapestries of every other avatar within every temple to have white, glowing eyes, alerting everyone in the avatar world that the Avatar had returned.

In Appa's Lost Days, Appa returns to the place he called home and saw Guru Pathik there. Eventually, Appa lost his anger and the enlightened Guru told Aang of his location and gave him a message to give to Aang. But he was imprisoned at Ba Sing Se by Long Feng.

In The Guru, Aang comes to the Eastern Air Temple to unlock his seven chakras but he left after trying to unlock his last one, blocking the chakra. Aang was then killed by a lightning bolt from Azula while attempting to unlock the seventh chakra, severing his connection from the state. Katara however manages 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 (considered canon) he restores his connection to the state, but has to unlock all his chakras again in order to use the state once again (though he could still meditate).

After the failed invasion, the gaang took refuge in the Western Air Temple and are currently still there.