I have a bit of a problem with the Earth Kingdom...their government setup is a bit...off. Each of the major Earth Kingom cities has it's own king? King Bumi of Omashu, the King of Ba Sing Se? Who governs the people of small taxes? Are they more like city states and the villages are just kinda on their own? Also, does the Dai Li have enough troops to defend the entire city from an invasion? Iroh attacked Ba Sing Se during his prime and was unable to break through their defenses. So did troops from Omashu assist? The whole setup kinda confuses me...it seems like the ununified Earth Kingdom should be doing far worse in the war then they are...

The Earth Kingdom is set up in a feudal system. Sure there maybe one king who rules all, but each city state has a ruler who runs their city/territory. One of the reasons why the Fire Nation is able to run around the Earth Kingdom countryside is because there is no united front. One territory ruler may not like his neighbor ruler so if the Fire Nation attacks their neighbor the ruler doesn't care, or may even side with the Fire Nation to get some gains. As for the Dai Li they are not really concerned with the Fire Nation. They are out for their own gains of power by running the city. When Iroh attacked Ba Sing Se the Dai Li in the city may not have been as powerful and the current king may not have been so aloof about the situation, so the Earth Kingdom guard was able to put up a better defense. Also remember the Fire Nation didn't have all the stuff it has now. Or as far as we know. Iroh was trying to siege the city like a castle and was only about to break past the outer ring late into his siege. But by then most of his troops where tired of fighting and upon finding out there was another ring they gave up and left.