Courage

This is what the Army has to say about it:

"All soldiers must be prepared for tasks that involve the use of controlled lethal force: to fight. They may be required to take the lives of others, and knowingly to risk their own; to show restraint, even when doing so involves personal danger; and to witness injury or death to their comrades but still continue with the task in hand. This requires physical courage, and soldiers will depend on each other for it. Moral courage is equally important. That is the courage to do what is right even when it may be unpopular, or involve the risk of ridicule or danger; and to insist on maintaining the highest standards of decency and behaviour at all times and under all circumstances. In the end this will earn respect and foster trust.

Courage - both physical and moral - creates the strength upon which fighting spirit and success on operations depend. It is a quality needed by every soldier, but it is especially important for those placed in positions of authority, because others will depend on their lead and respond to it."

The emphasis is mine. I think these are the elements than can be particularly difficult to show in yet are probably the most important.