Chapter XXIX of The Screwtape Letters, but you probably knew that already.
Cowardice, alone of all the vices, is purely painful – horrible to anticipate, horrible to feel, horrible to remember; Hatred has its pleasures. It is therefore often the compensation by which a frightened man reimburses himself for the miseries of Fear. The more he fears, the more he will hate. And Hatred is also a great anodyne for shame. To make a deep wound in his charity, you should therefore first defeat his courage.
Now this is a ticklish business. We have made men proud of most vices, but not of cowardice. Whenever we have almost succeeded in doing so, the Enemy permits a war or an earthquake or some other calamity, and at once courage becomes so obviously lovely and important even in human eyes that all our work is undone, and there is still at least one vice of which they feel genuine shame. The danger of inducing cowardice in our patients, therefore, is lest we produce real self-knowledge and self-loathing with consequent repentance and humility. And in fact, in the last war, thousands of humans, by discovering their own cowardice, discovered the whole moral world for the first time. In peace we can make many of them ignore good and evil entirely; in danger, the issue is forced upon them in a guise to which even we cannot blind them.
Fortunately, things are not nearly as bad now as they were when the book was written. But the basic gist remains relevant, methinks.