How fucked would the world be if...?
The Fire Nation attacked:
We saw this in the show, not fun for anyone
They have pretty advanced tech and they’re good at propaganda, and the whole genocide
Sozin’s comet puts the whole world at a disadvantage. The eclipse also only gives an 8 minute advantage to everyone else
Their biggest strength was their tech and timing, without it they would have failed
They’re good at war. That’s not a good thing
The Water Tribes attacked:
Waterbenders are pretty OP - especially when they figure out bloodbending
They have both of the poles as their territory, and the north is a stronghold, plus at least two chiefs to take out if you need to get rid of a leader/figurehead
I’m pretty sure the chief is elected, so you can’t even take out one family and be done with it. Plus that would imply at least the majority of the people are on board with the whole world domination thing if they’re electing warmongers
Advanced navy/naval warfare. You’re not gonna win a fight at sea if there’s waterbenders on the other side
The Earth Kingdom attacked:
They have the biggest territory, probably the most people, and if you’re on the ground, you’re vulnerable
Also at least two kings, so taking one dictator out is only 50% effective; Dai Li, brainwashing and propaganda could be used to get the Avatar on their side anyway
If Toph is involved, you can boost the threat level up to 100/10
When the pacifist nation decides the world is messed up enough that they’re taking over, you done fucked up
Their element is air. Good luck finding somewhere to hide without air
Also good luck escaping an airborne army with flying bison - especially because this is before any sort of war balloons or flying devices
They can bend air out of peoples lungs. Also Gyatso possibly probably killed the firebenders who attacked him in canon in this way and I can never look at that scene the same way again
Just thank the spirits they’re pacifists
In conclusion: the Fire Nation were probably the weakest conquerors, and Fear The Air Nomads