Air Nomad Genocide

"With Roku gone, and the great comet returning, the timing was perfect to change the world. I knew the next Avatar would be born an Air Nomad, so I wiped out the air temples. But somehow, the new Avatar eluded me. I wasted the remainder of my life searching in vain. I know he's hiding out there somewhere. The Fire Nation's greatest threat ... the last airbender."

- Sozin's memoirs about the genocide

The Air Nomad Genocide was an extermination campaign by the Fire Nation and is a pivotal event in Avatar: The Last Airbender. It was a genocide instigated by Fire Lord Sozin against the Air Nomads with the intent on eradicating the Air Nomadic people and killing the Avatar in the process as well as ending the Avatar Cycle. By doing so, the Fire Nation would eliminate its biggest obsticle and would be able to conquere the rest of the world.

Background
After Fire Lord Sozin had left his former friend, Avatar Roku to die at the hands of toxic volcanic fumes, he commenced his plans to spread the Fire Nation's "state of peace and wealth" with the rest of the world which plunged it into the Hundred Year War. Sozin knew however, that the Avatar was going to be reborn and that he was to be reborn as an Air Nomad and planned to stage a large scale attack on the Air Nomads once Sozin's Comet came which would grant Firebenders immense power over their enemies.

Due to the threat of war and how imminent it was, the Council of Elders at the Southern Air Temple had identified the young Air Nomad Aang as the new Avatar when he was only 12, which was unusual due to the traditional age of notification was at age 16. Gyatso knew that this meant that Aang was going to be sent away and his childhood was effectively coming to an end too soon. He said that Aang should be allowed to have a regular childhood, but the council refused and the High Monk, Pasang, demanded that Aang be sent off to the Eastern Air Temple to complete his airbending training, Aang overheard this conversation however and fled in the middle of the night with Appa. The two were caught in a storm and nearly drowned, though Aang used the Avatar state and the two were spared from drowing and the incoming genocide.

The genocide
By Autumn, the Fire Nation had amassed their armies and were now positioned at the four Air Temples, though it's unknown how they managed to locate them due to the Air Temples being built in secluded geographical positions with Aang only being able to make it to the Southern Air Temple by Sky Bison. Regardless, the temples were reached and the Fire Nation armies attacked them with each temple suffering a varying degree of damage from the more damaged Eastern Air Temple to the more intact Western Air Temple. The attack caught the Air Nomads by surprise and the Fire Nation maintained the element of surprise.

The Air Nomads were known for their pacifist way of life and philosophy, but that didn't stop them from holding their ground and put up fierce resistance against the invading Fire Nation troops. Evidence of this is shown with the remnants of fallen Fire Nation soldiers, especially the large pile of dead Fire Nation troops near the corpse of Monk Gyatso after his last stand during the genocide. The Fire Nation managed to press on however and eventually defeated and nearly exterminated the Air Nomads as a whole. All four temples were eventually taken and their inhabitants had all been killed, though some did manage to escape.

Following the end of the campaign, the Air Nomads had been nearly exterminated, but some did manage to escape the grasp of the Fire Nation. Knowing this, Sozin ordered the relics from the temples to be taken and used to create traps to lure in surviving Air Nomads. These traps were various caves in mountainous parts of the Earth Kingdom turned into makeshift residences. Spies then spread rumors of safehouses for surviving Airbenders across the Earth Kingdom (most likely occupied territories) and this had surviving Air Nomads lured in and then killed by Fire Nation assassins.