The sentence that develops the theme that a soldier often must harm other people to protect oneself is:
B. "He could not hope to capture that enemy; to alarm him would but send him dashing to his camp with his fatal news."
This sentence highlights the dilemma faced by soldiers where action must be taken to prevent a threat, even if it involves causing harm to another person. The urgency to protect oneself (and one's camp) is emphasized here, illustrating the conflict soldiers experience in such situations.