As stated by Hideo Minaba in an interview, war is the primary theme and setting of the game. To know the theme of Final Fantasy XII, it is first important to understand the impact of war on civilian population. For those who are fighting in a war, one must ask, "what are the people willing to pick up a gun and fight for?" Niccolo Machiavelli states that leaders of a country cannot pay a man enough to die for them. Men die in a war for something greater than non-economic: they fight for a flag, family, or perhaps their home. For the soldiers in Archadia, they fight for honor and chivalry. For the resistance in Dalmasca they have the home-court advantage, allowing them to have the determination to fight harder and longer.
Archadia's, or rather Vayne's, desire for a greater role in world affairs creates a conflict in which the people of Dalmasca are surpressed. His speech at the begnning of the game shows that he is willing to extend a warm and welcoming hand to the people of Dalmasca. This, of course, is nothing but a facade-- a cover up to fool the people to accept a forced change. The civillian population knew better since the destruction of Nabudis; the gravity of war was realized with the overthrow and assasination of their beloved King, thus the veil of unknowing lifted.
As for Balthier, did not care much for the war nor did he choose to fight for a particular side before he met Ashe, despite knowing the horrors of it. He criticized the insurgency calling them, "Boys with too much hate and too little experience", as he looks down at the poorly equipped corspes of resistance members lying on sewer floors. His attitude towards death itself is passive to an extent as to make somewhat ironic remark at Vaan, "Relax, it's just a corpse. Jump at every little thing down here and you'll wear yourself out". As much as Balthier would have liked to avoid it, evidence of warfare is left at ever corner he turns; it was much more than a coincidence that he participated in the fight. He is very well aware this war is morally wrong, but the fact that his homeland is causing the trouble conflicts with his inital decision, despite all attempts to disown Archadia. More importantly, the fact that his own father lent a hand in causing a few key conflicts catches Balthier's utmost attention. Perhaps he had not fully realized in the time before the game that war is much more than a nuisance, and his contribution would decide the outcome of it.