system
Given the following post, respond as if you're commenting on Reddit:
human
[Avatar: The Last Airbender] Long ago, the Four Nations lived together in harmony. Did they really? Was there truly harmony for all of history before the outbreak of the Hundred Year War?
gpt
"Harmony" does not necessarily equal "peace". There were roughly 10,000 years of the traditional "four nations" system before the Fire Nation ignited the Hundred Year War. That's an unrealistic time for there to be a perpetual peace (by comparison, our world is currently in what is known as the "Long Peace", an unprecedented period of peace between the great powers. This has been 75 years). Harmony needs to be looked at as more akin to "balance" in the traditional sense.

In the millennia prior to the HYW, there absolutely were conflicts between the Four Nations. The Earth Kingdom conquered land from the Eastern Air Nomads. The Northern Air Nomads conquered Earth Kingdom territory to establish the Northern Air Temple. The Fire Nation and Earth Kingdom fought for control of the Mo Ce Sea. Et cetera. But virtually all large-scale conflicts were internalized to one nation, such as the Unification of the Fire Nation or Chin the Great's War of Conquest. We can see this in the show, as Sozin attempted to establish several colonies in the Earth Kingdom prior to the official outbreak of the HYW that were quickly and relatively painlessly expunged by Avatar Roku. By and large, during this period people from all nations were free to travel without fear of persecution based on their home nation.

The outbreak of the HYW was different in that it was not entirely based on conquest. It began a war of extermination with the explicit goal of carrying out the genocide of an entire race. The Air Nation was all but wiped out in one massive blow. How could a member of the other two remaining nations hope to just "go and visit" the Fire Nation after that? 

The beginning of the HYW was akin to something like the outbreak of World War One. Before that you had things like generals riding out with their feathered caps and bright uniforms with these ideas of chivalry and honor, and then a few months in you had generals drawing up battle plans with the stated goal of killing as many human beings as possible in the shortest amount of time. The idea of "Harmony Among the Nations" was over, and if you ask some it still hasn't truly come back. With things like the United Republic illegally occupying Earth Kingdom territory, the independence of the Southern Water Tribe, the opening of the Spirit Portals, and the New Air Nation turning into something akin to a paramilitary force in recent years, the idea of true restored harmony seems like a pipe dream.