The Hundred Year War in Avatar: The Last Airbender was started when Firelord Sozin harnessed the power of a passing comet to strike a devastating first blow against the rest of the world. The comet greatly enhanced the powers of firebenders, which Sozin used to wipe out the Air Nomads in an attempt to break the Avatar cycle. Sozin also used the comet to begin his invasion of the Earth Kingdom and the Water Tribes.
Although Sozin's Comet marked the beginning of the war, there was another factor that caused the event to happen in the first place. The industrial revolution that the Fire Nation underwent just before the war was the true cause that motivated Sozin's invasion of the rest of the world. The Fire Nation's industrialization kept the war alive for a hundred years and prevented the nation from being overwhelmed by the combined might of the Water Tribes and the Earth Kingdom.
The Start of the Fire Nation's Industrial Revolution
During Sozin's reign as Firelord, the Fire Nation was experiencing a period of immense prosperity. This prosperity sparked a large number of technological advancements that greatly improved life for the citizens of the Fire Nation. The nation's industrialization required an increase in labor and resources, more than what could be found within their own borders. Just as in real life, this need motivated an increase in imperialism across the world.
Sozin claimed that he began his invasion because he wanted to spread the prosperity of his nation to the rest of the world. This was just a lie to cover up the true reason for the war. Sozin intended to steal the labor and natural resources of the other nations so he could increase the wealth and power of his own country. Before the war even started, Sozin had attempted to colonize Earth Kingdom territory and build a factory there before he was stopped by Avatar Roku.
If the Fire Nation had never entered their industrial revolution, there would have never been a need to gather more resources and labor. These resources sustained the Fire Nation and allowed them to continue developing new technology. Without this, Sozin would have never felt the need to invade the other nations. Industrialization created the need for war, which in turn helped fuel the industry. A self-fulfilling cycle was created that required the oppression of others to maintain it.
Without Industrialization, the Fire Nation Would Have Been Easily Defeated
Sozin's Comet gave the Fire Nation a huge head start in the Hundred Year War, but it was their technology that kept them in the lead. Before the Hundred Year War, most traditional warfare was fought by foot soldiers with bending abilities or handheld weapons. If this style of combat had continued into the Hundred Year War, the Fire Nation would have been quickly defeated.
In a traditional war, the Earth Kingdom and the Water Tribes have a clear advantage against a Fire Nation invasion. Both nations are much more familiar with their own territory than the foreign Fire Nation army. Waterbenders could easily sink any handmade ships that came to their waters, and earthbenders could disperse a troop of Fire Nation soldiers with ease. But the Fire Nation was not fighting a traditional war.
The technologies created from the industrial revolution gave the Fire Nation a competitive advantage in war. They had metal tanks that earthbenders couldn't easily destroy. They had steam-powered metal ships that could withstand the assaults of waterbenders. These war machines could move troops faster than the armies they were fighting and cause much more destruction. Ships and tanks that come from factories can also be more easily replaced than master-level benders that are killed or captured in combat.
The Industrial Revolution Allowed the Fire Nation to Always Be Innovating
For the entire span of the Hundred Year War, neither the Water Tribes nor the Earth Kingdom attempted to create new technology. These nations stayed stagnant and continued to only send out benders, who were at a severe disadvantage against the Fire Nation's machines. They never even attempted to develop new technology that would allow them to catch up with the Fire Nation.
The Fire Nation, on the other hand, was always coming up with a new weapon to use in the war. They developed tanks that could right themselves if they were knocked over. War balloons and airships armed with cannons and bombs were created in the last year of the war. These weapons almost won the Fire Nation the war when Ozai was planning on burning the Earth Kingdom to the ground.
The disparity between the Fire Nation and the rest of the world in technology was made the clearest when the Outer Walls of Ba Sing Se were breached. The Fire Nation had invented a drill that breakthrough Ba Sing Se's walls. The drill was metal, so the defending earthbenders couldn't bend it, and it had treads that allowed it to escape any hole it got caught in. The defenders of Ba Sing Se could only ineffectually throw rocks at it. If Team Avatar hadn't intervened, Ba Sing Se would have been conquered then and there.
