in the screwtape letters how does the enemy use war to his advantage

Basically war can consume the human spirit to turn to bad things. The shadow of cruelty and death is always upon people in war.

We may hope for a good deal of cruelty and unchastity. But, if we are not careful, we shall see thousands turning in this tribulation to the Enemy, while tens of thousands who do not go so far as that will nevertheless have their attention diverted from themselves to values and causes which they believe to be higher than the self. I know that the Enemy disapproves many of these causes. But that is where He is so unfair. He often makes prizes of humans who have given their lives for causes.