War is ugly. Physically, psychologially, emotionally, socially, morally, etc.
If we must wage war, let us wage war in such a way as to break the enemy's will to ever fight again. Show no mercy. Utterly shame and decimate the enemy if necessary. As long as people have sinful hearts full of pride, lust, greed, etc., people will be moved by their pride, lust, greed, etc. to wage war. Thus to defeat them and best ensure no future war with them one may need to completely humble and shame their proud hearts. I believe this is more or less the argument scholars like Victor Davis Hanson and Donald Kagan have made in books like The Western Way of War, The Father of Us All, and On the Origins of War and the Preservation of Peace.
That is, think not Germany after WWI. But think Germany after WWII. After WWI, Germany was defeated but unbowed. The German government surrendered even though German troops were still physically occupying territory on the Western front and even though Germany had won on the Eastern front against Russia. So Germany sought to avenge their defeat at an opportune time. But after WWII, Germany was defeated and bowed to their knees. They knew they had lost. Soviet armies crushed Germany from the east and Anglo-American armies from the west. Germany lay in ruins.
This seems to be the best way to ensure a lasting peace. And, as a friend points out, this can potentially save more lives in the long run.
I suspect this is one reason why God commanded the Israelites to show the Canaanites no mercy if they insist on war. If it's going to be war, then let it be all-out or total war such that the enemy will absolutely know he is defeated, have his will to fight broken and shattered to pieces, and never seek to fight again.
I realize this sounds harsh. But we live in a fallen world. These are the sorts of harsh realities we're often forced to face. These are the difficult choices we're often forced make.
