system
Given the following post, respond as if you're commenting on Reddit:
human
[TLA] How did the Fire Nation fight the war for a hundred years without conscription?

Manpower had to be a serious issue for the Fire Nation; the Earth Kingdom is huge, after all, and the army had been taking losses against fierce resistance for decades, but it seems that the Fire Nation military is all volunteer.  How did they manage to get enough people to sign up?
gpt
A martial culture where service is an honour. Painted as the highest duty and calling of all loyal sons and daughters for the Fire Nation. Ruled by a single family, playing off (or executing) factions against one another, they could act decisively as required.

They really didn't NEED that many people. The advance across the earth kingdom wasn't the meatgrinder of the trenches you'd see in WW1 and 2, but a slow, steady affair with regular consolidation, careful logistics and reinforcement. Colonies were established in the wake of the advance, providing additional manpower. Troops were regularly brought in from the home islands or other duties - naval or otherwise. The incredible offensive power of firebending meant that casualties were low. Most of their navy and war machines were metal and relatively high technology. Force multipliers abound.

Add in the Earth Kingdom - while the Fire Nation was politically unified, with martial traditions such as agni-kai holding legal weight, the EK was not. A far flung system of provinces with local lords and kings who were essentially autonomous. Their military was undertrained, outmatched technologically and large scale offensives were unknown because of this political divisiveness.