We fight for reasons that are, for the most part, essentially nonsensical, even when the justification seems plain and straightforward. Two kingdoms, one upriver, one down river. The kingdom downriver sees the water arrive befouled and sickly, filled with silts and sewage. The kingdom upriver, being on higher land, sees its desperate efforts at irrigation failing, as the topsoil is swept away each time the rains come to the highlands beyond. The two kingdoms quarrel, until there is war. The downriver kingdom marches, terrible battles are fought, cities are burned to the ground, citizens enslaved, fields salted and made barren. Ditches and dykes are broken. In the end, only the downriver kingdom remains. But the erosion does not cease. Indeed, now that there is no irrigation occurring upriver, the waters rush down in full flood, distempered and wild, and they carry lime and salt that settles on the fields and poisons the remaining soil. There is starvation, disease, and the desert closes in on all sides. The once victorious leaders are cast down. Estates are looted. Brigands rove unchecked, and within a single generation there are no kingdoms, neither upriver nor downriver. Was the justification valid? Of course. Did that validity defend the victors against their own annihilation? Of course not.
'A civilization at war chooses only the most obvious enemy, and often also the one perceived, at first, to be the most easily defeatable. But that enemy is not the true enemy, nor is it the gravest threat to that civilization. Thus, a civilization at war often chooses the wrong enemy.