I don’t understand the reason for why it may be impossible for modern warfare to be impossible to be a just war. It is arguably easier for a modern war to be just than it was in the past. The reason? Communication & control. In past centuries you were accepting the inevitable reality that war would be teeming with chaos. You’d have armies of peasants being conscripted by independent vassals, who in turn swore a fealty to a king, who was basically a head vassal among them. They’d all gather together in a discombobulated mass of different coats of arms and wage a war, often times with different nobleman each being largely independent in controlling their different territories. There was vary limited means of controlling the masses of conscripts. Treaties did not bear the accountability that they do in our far more connected & organized modern world. Exchange of prisoners and the acceptance of surrender was a **** shoot. Trials and punishment towards soldiers that stepped out of line or sometimes did absolutely atrocious deeds were often forgotten in the chaos, overlooked, or even applauded. War was chaotic and it was an inevitability reality even for a war that seemed to abundantly meet the perquisites of a just war.
The expansion of control, checks & balances, and organizational structure have made a modern military more accountable to the public than they have ever been before. Major decisions to intervene in an event are often done with a large amount of international cooperation and discussion. Civilian casualties have decreased enormously compared to many of the wars waged in the medieval ages. 35+ million people died in the Chinese An Shi Rebellion, which was over a thousand years ago. That makes the ~70 million that died in WW2 in the population of the modern world almost a drop in a bucket in comparison, and WW2 is frequently believed to be the absolute most brutal war in history. Not by a long shot.