The term “war crimes” gets used a lot when people criticize characters from the Fire Nation. Most of the time, it seems they apply this to any situation in which the Fire Nation acts as the aggressor in an armed confrontation, such as:
Laying siege to Ba Sing Se
Burning huge swaths of land in the Earth Kingdom
Hunting the Avatar (Yes, he’s a 12-year-old kid, but he also functions as a weapon of mass destruction.)
Unfortunately, those things are not war crimes according to the UN definition of the term. Those things are just…war. It could be argued that wherever there is war, there will be war crimes because war itself is fertile ground for all kinds of human rights violations. But according to the strictly legal definition, the simple act of fighting in a war or commanding troops is not a war crime.
However, the show does provide clear cases of actions that would be considered war crimes in our world, such as:
The genocide of the Air Nomads (”The Southern Air Temple”)
The capture, torture, and killing of noncombatants from the Southern Water Tribe (”The Puppet Master”)
Dressing prisoners of war in Fire Nation uniforms and putting them on the front lines, unarmed (”Zuko Alone”)
But there’s another thing the Fire Nation does that most people don’t pay attention to: “conscripting or enlisting children under the age of fifteen years into the national armed forces or using them to participate actively in hostilities" (source, p. 6)
Azula is fourteen years old, dear reader. And for all we know, she may have been closer to thirteen than fifteen when Book 2 starts.
So, Ozai permitting (ordering?) Azula to act as a combatant in the war? War crime. Sending her to capture or kill Iroh and Zuko? War crime. Letting her act as a decoy on the Day of Black Sun? War crime. Allowing (commanding?) her to pursue Team Avatar to capture or kill them? War crime.
See, the problem with using underage children as combatants is that they lack the maturity and judgment to express consent or fully understand the implications of their actions, and their adult commanders often push them to commit atrocities (source). They’re also easily intimidated and indoctrinated, which makes them highly desirable recruits for military leaders who have zero scruples.
We see this clear as day in Azula. She’s been fed Fire Nation propaganda since the day she was born, and she spent several years by herself under Ozai’s thumb. When she arrives in Omashu, she doesn’t rename the city after herself. She names it after Ozai. She doesn’t take over Ba Sing Se for herself. She does it for the Fire Nation. Furthermore, she’s eager to do these things for Ozai’s approval. In fact, her actions in the war are so entwined with Ozai’s approval that we have no way of knowing exactly how culpable she is for them.
But, y’know, she’s just evil or crazy or whatever.