system
Given the following post, respond as if you're commenting on Reddit:
human
[ATLA] Why hadn't the fire nation invaded the northern water tribe before the return of the Avatar with a navy similar in size to that which Admiral Zhao mobilized?
gpt
The fire nation, while expanding, still has finite resources. During the 100 year war, they focused most of their efforts on expanding into the Earth kingdom. 

It wasn't until Zhao and the Avatar that the northern water tribe became a priority.