The Fire Nation virtually wiped out any major opposition from the Water Tribes through the systemic Southern Water Tribe raids. These attacks put the Southern Water Tribe in no position to fight and prompted the Northern Water Tribe to isolate itself from the rest of the Avatar World.
That said, the Water Tribes did have a fighting force in Hakoda's fleet of Southern Water Tribe warriors. Their operations, however, were more stealth-based and were only minor occurrences in an otherwise global-scale war.
On top of that, the Fire Nation and Earth Kingdom were the most war-readied forces in the Hundred Year War. The Fire Nation being technologically advanced to some extent and the Earth Kingdom possessed a great defense.