>>712625197
what are you talking about?
the earth kingdom is more than its capital, the fire nation spread out far through its territory.
the southern water tribe was basically already under its control and they almost crushed the nothern water tribe.
They were pretty close to winning it all and just stalled for both the solar eclipse to pass and Sozin's comet to arrive.

Though its not very clear what they did for most of the 100 years, maybe the war didn't escalate right after wiping the airbenders. I'm not sure if it was explained why they targeted them first, other then to show off how evil they are.
Maybe air bender can make someone suffocate from a distance or something that worried them, despite being pacifists.