The central theme of Starship Troopers is duty—duty to one’s fellow soldiers and, ultimately, duty to all of humanity.

Johnnie Rico listens doubtfully to his History and Moral Philosophy teacher, Mr. Dubois, who lectures endlessly about the importance of duty as a chief attribute of good leadership. Later, as a cadet officer in the Mobile Infantry, Johnnie attends another Moral Philosophy class that elaborates on the concept of duty as the highest calling. The classes teach that people become virtuous when they place others above themselves: “[…] a juvenile becomes an adult when, and only when, he acquires a knowledge of duty and embraces it as dearer than the self-love he was born with” (126). In that sense, the purpose of life is to be of service to one’s fellow humans. The highest duty, then, is to risk one’s life to protect others.

Duty extends as far as one’s own species and no further. Civilizations that try to limit their growth, for example to save resources or prevent overpopulation, will be overrun by any species that feels no such compunctions. In interstellar space, humans who colonize other planets face competition from alien species who want those same worlds for themselves.