I’m so glad you asked! A caveat before we get going – ATLA is a children’s television show and is not designed for geopolitical analysis. That said, I do think it’s possible to do it with a bit of inference and squinting at maps. So, my argument is that the Fire Nation began the Hundred Year War (HYW) as a maritime campaign of colonial acquisition and geostrategic defence, but their war goals steadily totalized – especially in the late war.
To expand that out a bit, I’d identify three core phases to the HYW from the Fire Nation perspective:
1. A largely littoral effort in the early parts of the war to seize outlying Earth Kingdom territory.
2. A stalemate imposed by geography and distance.
3. Attempts to break that stalemate through various escalating means, culminating in the Sozin’s Comet attack.
To explore this, let’s go back to Sozin. Why did Sozin start the HYW? We see on screen that part of it is a ‘civilizing mission’ (quotation marks very much deserved) to share the riches and wealth of the Fire Nation with the rest of the world. Now, that’s him explaining his cause to Roku, so he’s certainly couching the effort in a particular rhetorical stance – but the idea of some sort of ‘Hotman’s Burden’ (to bastardise Kipling) would make sense from what we know about Fire Nation culture.
But it’s not just that, of course. Part of it is simply colonial expansion – the western coasts of the Earth Kingdom were up for the grabs, so Sozin took them. But I think, perversely, the war can be better understood as a pre-emptive forward defence. Why? Because a strong, united Earth Kingdom is the Fire Nation’s worst nightmare.
To take a brief trip into British history, one of the core tenets of British foreign policy from the 1500s onwards has been to avoid a single country dominating the coasts of western Europe. If this were to happen, invasion attempts against Britain would be in the offing – the Spanish tried this, Napoleon tried this, Hitler tried this.
I think Sozin thought similarly. We know from the Kyoshi books that the Fire Nation takes the threat of the Earth Kingdom very seriously – Rangi notes that the Fire Army plans and trains for a war with them, on the order of three centuries before the outbreak of the HYW. However, this was not a major concern because the Earth Kingdom was in frankly shambolic straits. It could not police itself, individual powerbrokers operated fairly independently, and the risk of some sort of invasion of the Fire Nation by the Earth Kingdom was very low.
However, then Kyoshi (who, incidentally, is awesome) comes along and starts fixing the Earth Kingdom. She scours daofei and defeats warlords like Chin. The Earth Kingdom, under her watch, starts to stabilise – and centralise, too (for example – refugees in the HYW were expected to run through a fairly complex bureaucratic system to get to Ba Sing Se. That points to quite an advanced level of governmental development).
If I’m a Fire Nation strategist at this point, I am starting to get seriously concerned. A centralised Earth Kingdom would have access to far more resources than I ever would. In addition, while the Fire Nation enjoys a technological advantage, it’s not out of the question to assume that the Earth Nation would begin to catch up – certainly we see in the post-HYW era that industrialisation is very swift in the western Earth Kingdom. I need to either attack now while I’ve got a chance or watch while my neighbour becomes a continental superpower able to impose demands or conquer me. Or try diplomacy but let’s be honest, the Fire Nation’s government wasn’t set up culturally or institutionally for that.
(Incidentally, this is almost to a tee German logic about the First World War – Russia was developing rapidly, and the German military felt that they were going to go to war at some point; and so the sooner, the better).
And so, Sozin decides to pull the trigger and invade. This also necessitates the removal of the Avatar as a precondition, hence the timing and scope of the – utterly barbaric – genocide of the Air Nomads. But this invasion appears limited in scope.
Our evidence base on this is exceptionally patchy, but when we look at maps of the Fire Nation’s advances, they are largely confined – even after a century of fighting – to the western edge of the main continent. They have a secondary front down south in Chameleon Bay and have at points lunged eastwards to positions like Pohaui, but their landwards expansion isn’t very big. Omashu is very close to the Fire Nation and didn’t fall; and General Fong’s fortress is similarly nearby, albeit to the north. Gaoling, too, is untouched by the war.
My thinking is that this is partly a matter of simple logistics – sustaining armies advancing deeper into the enemy heartland, away from sea lines of communication (SLOCs) is very difficult. But I also think this was strategic intent. The Fire Nation can’t govern huge swathes of the Earth Kingdom. Even their very limited footprint at the end of the HYW is unruly. Rather, Sozin hoped to bite and hold a small part of the coast, demonstrate the weakness of Ba Sing Se, and hence encourage other parts of the Earth Kingdom to splinter. This would end any threat to the Fire Nation and net some neat and prosperous colonies into the bargain.
But they didn’t. The Earth Kingdom stayed more or less coherent – see, for example, all their soldiers being equipped similarly which points to state-run arsenals – and fought back. So now the Fire Nation is left with a real problem. They can’t advance too far from the coast without facing major challenges, but they also can’t defeat the enemy without such an advance. They could retreat, but honour and national prestige means they can’t. So, they’re stuck in a stalemate through much of the war.
By the late-war period, and as Iroh’s generation – the second generation of military leaders born into the war – comes to prominence, patience is running out in the Fire Nation. Decades have passed and the Fire Nation is no closer to victory. Negotiation is unthinkable in a militaristic and honour-bound society. So, the obvious choice is to escalate and break the stalemate by main force.
Iroh’s first up to bat, waging a conventional campaign against Ba Sing Se. He does well – very well – clearing the path to the city and breaching the Outer Wall. But the tyranny of distance, logistics, attrition, and the loss of his son prevents a final push to victory. Zhao turns to spiritual means, hoping to kill the Moon and break the enemy will (or he’s just a bit insane, it’s not entirely clear). Azula and Ozai opt for technological solutions – a giant drill, and then a fleet of airships.
We are also seeing, increasingly, the barbarisation of the war. The Fire Nation clearly look down on the Earth Kingdom, and harbour supremacist sentiments going all the way back to the Age of Kyoshi. But Ozai brings this to the forefront, no doubt informed by decades of tit-for-tat atrocities and pointless, bloody stalemate; imperial powers and resistances to them both have quite lax rules of engagement around civilians. This is what allows Ozai and Azula to propose genocide as an ultimate war winning tool – and what turns a large enough coalition against the Fire Nation that they lose.
So…yeah. There you have it. Fire Nation grand strategy starts as quite sensible, but under the pressures of war and tyranny becomes an increasingly total and barbaric effort which culminates in the attempted extermination of the entire Earth Kingdom, vis a vis the mere countering thereof.
PS: From this perspective, the creation of the United Republic as a pro-Fire Nation power on the western coast of the Earth Kingdom represents a strategic victory for the Caldera. No wonder Hou-Ting and Kuvira are both so revanchist!