In the United States, we tend to do our war from afar ("Safe! Rare! Legal!") and so have long believed that we are in a time of peace, relatively speaking. We also have the rather naïve and conceited belief that we could never, as a people, devolve into the savagery we see in other nations' civil wars or ISIS. Alas, we will someday put ourselves to shame.
In looking at the Old Testament, we see the nature of war: brutal, deadly, and often godless. Petty and unjust. But, even with that said, we must remember that it is God Himself who first formally declares war, based on the deceptive provocation of His enemy: "I shall put enmity between your seed and her Ssed..." (Gen. 3:15). However, as should quickly become apparent, only God Himself is able to justly declare and prosecute war; even His holy servant Moses sometimes takes His commands and makes them unjust (for example, when Israel sins with Midian at Peor in Num. 25, the command is to "harass" them -- by the time we reach this in chapter 31, Moses commands his troops to kill all the males).
How, though, do we know that the war is just? Just War Theory can function as a guide, but there is something more. In the OT, the prophets often held the role of advisor to the political leaders (the judges themselves were often a combination prophet and magistrate), as they had stood in the Presence of the Just One and could definitively exposit His word on the matter. What about today, though? Who are the prophets we can consult?
My mind is directed towards places like Mt Athos in Greece, a place where men dedicate themselves to communion with God and so, sometimes, become wise. What would they say? My thought is that they would tell us the true nature of war is not against our own flesh-and-blood, our fellow humans, but against spiritual wickedness in high places. Even our land-based military exploits are guided by those beings who wish to see us tear one another apart.
This raises an important and practical question: what if we actually devoted ourselves to prayer over the issues that cause us to war with one another? Done trust the Spirit to work for reconciliation? We must remember, of course, that reconciliation of enemies takes the shape of a Cross (Rom. 5:10) -- our passions, individually and corporately, must be put to death. Even so, sometimes we may find ourselves needing to intervene or interpose for the protection of the weak, the widow, or the fatherless: we must do so with tears of lament over the damage to our fellow image-bearers and to the unavoidable fracture of our own souls.