@Rixile:
Methinks that the fire nation finally punctures the inner circle somehow (don't really know) and Aang will have to rally the people in some way. Maybe. This is all just crazy speculation mind you.
At this point, I think it's really just if they break through the Outer Wall. After that, the Fire Nation would simply be able to starve Ba Sing Se into submission; the city is in such bad shape right now that the reality of war would result in it falling apart. The lower classes would probably revolt outright as it's debatable as to how much worse life would be under the Fire Nation, especially if the food supply runs out.
So, Great Wall falling equals war over if Aang and company don't manage to seal the breach again. The Fire Nation managed a breakthrough once, but Lu Ten apparently died in the process or shortly after which prevented Iroh from exploiting the situation.
As far as the Wall goes, the easiest way would be to simply attack it directly with war balloons. If they can drive the Earthbenders back long enough for their tanks to scale the Wall, then reinforcements can begin pouring across.
@yokaiforte:
Well, seeing as Ba Sing Se will probably soon be the only earth kingdom city not controlled by the fire nation,
I'm pretty sure that it already is; Omashu and Ba Sing Se were the last of the major Earth Kingdom cities not already captured by the Fire Nation.
Unless the Fire nation feels like picking at the Water tribes which are situated in only two locations and could probably easily beat them down during a New moon,
Three actually, though I doubt that the Fire Nation knows that. Anyway, Zhao was an idiot by attacking when he did and the South Pole tribe would be extinct/occupied if any other Fire Navy ship other than Zuko's had been nearby when Aang accidentally revealed his return to the world. Their fighters (minus Sokka) are all in the Earth Kingdom, which is probably the only reason that the Fire Nation hasn't landed troops by sea. Their Navy appears to be quite a bit superior to that of the Earth Kingdom judging by the refugee ship and General Fong's statement to the effect that the Earth Navy attacking the Fire Nation would be suicidal.
I'm not sure what the current situation of the Fire Navy is, but I seem to recall hearing that they lost hundreds of ships at the North Pole. So unless Zhao's fleet was relatively insignificant, I doubt that they have any designs on the Water Tribes as serious damage to the Navy would prevent any real assault on the Northern Tribe and they might not even be aware of the location of the Southern Tribe.
If nothing else, they've probably deferred them until they finish off the Earth Kingdom.
they are likely to lead another seige on the city, only with all their forces, so that it doesn't take 600 days to make it past the wall.
I really don't know that it would take that long regardless of the numbers they send for the reasons I listed above. A breach in the wall would be hard to hide from the citizens; when they find out, that's it.