Thinking human beings not consumed by slogans and fanciful imaginations will realize that humane war is an oxymoron of grand proportions. War is never humane though unfortunately many times necessary. There is always a moral Hobbseian choice involved in going to war or defending one’s self in a war begun by others. War involves the deliberate killing of other human beings. By any stretch of imagination, all things being equal, this is not really humane behavior. But many times it is necessary behavior in order for any sense of ultimate humanity to survive that war comes upon humanity even if it is not humane. It is difficult to imagine how civilization as we know it would have survived the twentieth century without the war against Germany and Japan being ruthlessly prosecuted. The United States is in the midst of again waging a ruthless war against the Moslem terrorists in Afghanistan and Iraq and clandestinely in Pakistan. The methods being used to pursue that series of wars are hardly humane but again unfortunately they are necessary if Western civilization is to somehow survive. The hard choices that reality forces upon moral humans oftentimes lead to seemingly immoral behavior. But none of this can e viewed in isolation or in a vacuum. Thus the question of being humane and/or moral becomes one that only can be judged on a relative scale. And therefore there goes any sort of absolutist judgment on wars and the methods used to pursue those wars to a successful conclusion.
The goal of Judaism and Jewish life is the achievement of personal and national peace and harmony. Yet the Torah does not advocate radical pacifism. The Torah provides for the concept of milchemet mitzvah - a war that is an absolute necessity such as the war against Amalek (the epitome of evil incarnate in Torah terms) or a war of self-defense and self-preservation. The Torah also provides for wars that are titled milchemet reshut - wars that are permissible even if on the surface the war is not currently deemed necessary in the eyes of many. The Torah does not provide a guide as how to conduct the wars. It does allow for the complete destruction of evil such as Amalek and there are also provisions for making peace and accommodation with the enemy before the beginning of hostilities. But once the war begins there apparently is little room for a humane war, simply because there is no such animal. And the criticism of outsiders that these types of wars are unjustified because they are somehow not humane is unrealistic if not completely cruel and unwarranted. The Torah is not blind to the existence of evil in this world. It teaches that evil must be countered and fought. Since evil fights its battles in an evil fashion - certainly not in a humane fashion by any definition of the term - fighting a war against evil inevitably must fall short of being a humane war. That is simply the harsh reality of living in this imperfect, dangerous and mostly evil world.
The Torah nevertheless impresses upon us that war takes its spiritual and moral toll, even on the side that is justified in resorting to armed conflict to save itself and its way of life. King David, probably the most famous and justified warrior in Jewish history and who finally vanquished Israel’s enemies in the Land of Israel after four hundred years of conflict there, nevertheless was not allowed to build the Holy Temple in Jerusalem because of his participation in wars. Wars are necessary at times but they are never pleasant or humane. There is therefore always a penalty involved in having been engaged in fighting wars. But there are many things and situations in life that are a lose-lose situation. Then the choice is what is the greater loss and what will be the minimum loss involved. In the struggle against evil that always threatens to engulf us the lesser evil is always to confront it and defeat it. The confrontation will never meet the artificial constructs of being humane in conducting a war to outsiders or to pious pacifists, many of whom have built-in prejudices to begin with. We should always attempt to be careful not to become a cruel people. The Torah promises us that even though we will have to engage in wars, the Lord will send to you the capacity for mercy so that you will never become a cruel people. And I imagine that is the most that can be expected in the conduct of war - as humanely as possible.