In canon, there's a large implication that part of the reason why Arcadians are as paranoid and xenophobic as they are is due to the loss of the plane's third layer, even if they're not exactly certain that the layer is gone (all they seem to be aware of is that not as many people are reaching Nemausus). As a result (again, implied) they keep on a constant lookout for chaotic and evil individuals. They have to bring to justice those who disrupted the order of the plane.
So, what happens if the Arcadians become aware that the loss of their plane was due to the actions of the Harmonium? Would they try evicting them from the plane? How would the Harmonium react? Would they fight to keep their place on Arcadia, or would they give up their chosen planar homeland?
I've always likened Arcadia to the city-states of Ancient Greece. Each has its own government and distinct culture; effectively, their own way of forming an ideal nation. They're reasonably amicable toward each other -- what works for Athens doesn't work for Sparta, and so on, but there's no reason for one to try and conquer the other.
Then Persia comes along and tries to mess things up.
I'm not saying the Persian Empire is to the Greek city-states what the Harmonium is to the Arcadian nations. Well, not exactly. But there might be enough parallels for some spectacular imagery. The Harmonium would probably be outnumbered in terms of soldiers on the battlefield, for example, and would be forced to find "unconventional" means of evening the odds. Or maybe the Harmonium could make an outright attempt at conquering Arcadia. It could be the first step in proving (to themselves as much as the rest of the 'verse) that their way is the right way, and that they can, in fact, make everyone conform to it.
I think the Arcadian nations (most of them anyway) would form a temporary alliance to expel the Harmonium, which would force the Harmonium to fight a defensive war or to marshal their forces and go on the offense. They certainly wouldn't give up, as I see it, of only because they "know" they're in the right.
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