lips like galaxy's edge

Zestial print available via Streamily April 4, 2024. Signed by James Monroe Iglehart. Art by Onyx_Superbia.

[source] [source 2]

Sozin leaves Roku to die in the lava so his ambitions will go unchecked

Twelve years later, he uses Sozin’s comet to eradicate the Air Nomads

(As per the comics, survivors of the initial attack are lured into traps by using sacred air nomad artifacts as bait and picked off one by one)

Twenty to forty years after Sozin’s comet, the Fire Nation starts raiding the Southern Water Tribe

(The tribe that the next Avatar in the cycle, Avatar Korra, is from)

Waterbenders are captured and taken back to the Fire Nation to be imprisoned instead of being killed

(Hama and Gran-Gran, both in their eighties, are implied to be the younger generation hurt by the original raids)

Eighty to ninety years after Sozin’s comet, the fire nation builds an off shore prison designed for housing earthbenders

So therefore we can conclude:

Sozin never discovered exactly who the Air Nomad Avatar was, but after several decades assumed them to be dead

Avatar Kuruk was from the North, so the Water Tribe Avatar must cycle between the northern and the southern tribes

Therefore, Sozin and/or Azulon targeted Southern waterbenders because they thought one of them could be the next Avatar

Waterbenders were kept alive as long as possible in order to keep the Avatar out of their business

As the waterbenders began to die off, work was begun on an experimental prison to house new potential candidates for the Earth Kingdom’s Avatar