 Commentators have argued that Heinlein's portrayal of aliens, as well as being a reference to people in communist countries, invokes the trope of a return to the frontier.The concept of the frontier includes a social-Darwinist argument of constantly fighting for survival, even at the expense of indigenous people or, in the case of "Starship Troopers", of aliens.Heinlein suggests that without territorial expansion involving violent conquest of other races, humans would be destroyed.