The opening sequence of Avatar: The Last Airbender begins with the history of the four nations — the Water Tribe, the Fire Nation, the Air Nomads, and the Earth Kingdom — living in harmony before the Fire Nation became hostile to its neighbors. Fire Lord Sozin instigated the war and proceeded to find the new and reborn Avatar following the death of Avatar Roku. When Aang disappears for 100 years, most people assume he's gone for good and focus on dominating the Earth Kingdom and Water Tribes, while a select few (Zuko) continue to seek him out the Avatar.
Now, an intriguing new theory offers a compelling explanation for the Fire Nation's fixation with waterbenders before and during Avatar: The Last Airbender.
Reddit user LDM123 suggests the Fire Nation constantly pillaged and imprisoned members of the Southern Water Tribe because they originally thought Katara was the new Avatar. The Last Airbender notes that the Fire Nation had killed all of the airbenders (including the Avatar) and so they went looking for the next incarnation, a waterbender, next.
The post has since been removed from the Fan Theories subreddit for unclear reasons, but here's the gist of it.
The Avatar cycle is based on the four seasons. When one Avatar dies, their successor is reborn into the next nation. We know Aang is an airbender and The Legend of Korra later confirms that the next Avatar in the cycle is a waterbender. So it makes sense that the Fire Nation would look there if they thought the airbending Avatar was dead.
The show itself also offers plenty of evidence to support this theory. In Season 1 of the Avatar: The Last Airbender, we saw that the Southern Water Tribe’s population had depleted over the course of the Hundred Year War, and Katara was the last waterbender the tribe had seen in some time. Why would the mighty Fire Nation focus their military might on a small tribe living on the edge of the world?
It’s unlikely the Fire Nation would go looking for Katara specifically, but The Last Airbender Season 3 confirmed that the Fire Nation was on the hunt for waterbenders, rounding up and imprisoning anyone left (to be fair, we see them do the same to earthbenders in an attempt to solidify firebender supremacy).
Then, halfway through Season 3, flashbacks reveal how Katara and Sokka’s mother died: a Fire Nation soldier killed her, believing her to be a waterbender (she lied to protect Katara). It’s even possible that’s why Prince Zuko went south in search of the Avatar. Perhaps he also believed that they were a waterbender before he saw Aang for himself.
Conversely, it's possible Fire Lord Sozin was fully aware that the next waterbending Avatar hadn’t been reborn yet. A moderator commenting on the original post posited that the Fire Nation was only interested in sending ships to the Southern Water Tribe because they knew it would likely be Aang’s first stop since he came from the Southern Air Temple.
Indeed, waterbending was the first ability Aang learned on the show. What’s more, knowing that the Southern Air Temple was closest to the Water Tribe logically meant that the Fire Nation would stake out the area. It’s a strategic move for the militaristic Fire Nation, and Sozin and was known for thinking ahead.
But Avatar: The Last Airbender makes it clear that most of the world thought Aang was long dead. And if that's the case, it makes sense that the Fire Lord would focus on finding the next Avatar instead (even if he also sent his loser son on a wild goose chase to capture the Airbender). Of course, that's not what happened, but if you want a taste of what a waterbending Avatar looks like, there's always TLA's sequel series, The Legend of Korra.
Avatar: The Last Airbender is currently streaming on Netflix.