Eh, the only thing changed in my Bizarro Avatarverse is the nations they are born in not their personalities. Because the Water Tribe has such jerks as Ozai it isn't exactly a good nation and the Fire Nation having guys like Hakoda, Pakku and Chief Arnook isn't exactly bad either. The way I conceive it the Water Tribe with the likes of Souzen and Ozai at the helm desires to become a player in the world stage. They admire the Fire Nation's industry but lacking in the resources have resorted to attacking Fire Nation ships to acquire resources and people with the technical knowledge to jump start their industrialization. The Fire Nation asks the Earth Kingdom/Northern Air temple for terrorism but as the Earth Kingdom is getting paid off by the Water Tribe and the Air Nomads are isolationist they refuse. Fed up by the lack of response, the Fire Nation initiates a blockade of the Water Tribe anyway. The Earth Kingdom issues the Fire Nation a warning to cease and desist their activities in EK waters or it will be considered an act of war. The Fire Nation refuses thus kicking off the 100 years war we are so familiar with.