Bastok supposedely had the most powerful navy fleet in the world, but the Shadow Lord told the sahagin that they were polluting the oceans(which is probably not untrue). The sahagin sank the entire fleet. That fleet could have saved Tavnazia.
In the end the only army that actually fought the sahagin were the mithra who were the only ones sailing back and forth from the south.
The antican were bought by the Shadow Lord, they had the biggest army in the world. Bastok terrorized prefered to crush its own mines to cut them off from the midlands. If antican joined the war Bastok would have been destroyed, which would have meant the loss for the allies in the war.