system
Given the following post, respond as if you're commenting on Reddit:
human
[avatar] How did Sozin Know?

 

"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."--Sozin

So How did Sozin know he missed the Avatar? The FN's subsequent attacks on the Southern Water Tribe would seem to have been the natural confirmation that he had succeeded in destroying the airbender incarnation. What made him "know" that his mission failed?
gpt
We're led to believe that they new Avatar is born immediately on the death of their predecessor, which Sozin personally witnessed. So he knows how old the Avatar is supposed to be. If no one goes into the Avatar state doing the genocide, then you round up all the eleven year old kids and go Darth Vader on 'em.

Really explains why Gyatso killed so many of the soldiers.