This poem is a rather sardonic and cynical take on war; some might argue it is a realistic, rather than idealistic viewpoint of war. In fact, it contrasts the more idealistic, brave, glory parts of war with the harsh, violent, horrific realities of its impact on men.
It starts with a bishop trying to give comfort to men who go to war--he states optimistically that war will "change" all men who fight in it, and then lists all of the positive ways it will change them. He states that they will have "challenged Death," "fought in a just cause," and "lead the last attack on Anti-Christ." All of these things are things that the men should be proud of, that will change them for the better for the rest of their lives. The Bishop represents that attitude that war is a glorious thing, that war shows how brave and noble one is, and that war is the thing that keeps the entire world safe. And, while true on some levels, in the next stanza, Sassoon contrasts that war-glory with the realities that exist.
Men who have gone to war agree with the Bishop that war does change men, but certainly not for the better. They list all of their war wounds: amputation, blindness, syphillis, shot lungs, etc. The front these wounds as definite evidence that the Bishop is right that change occurs, but not in the way he was stating. The bishop's rather inadequate and pale response to this awful reality is an ambiguous and dismissive one. He merely states, "The ways of God are strange!" This answer, which feels more like a cop-out than a real explanation, shows that even the Bishop, a man of god, is left without answers in the face of the brutal realities of war. When faced with the true horrors of war, all of the brave, idealistic glory feelings die, leaving behind only the violence, despair and death.
I hope that those thoughts helped a bit; good luck!