In December 1958 Heinlein took Podkayne Fries off his agenda: A hundred pages into the manuscript, it just was not coming together for him. He must have been mulling over the intertangled notions of freedom and responsibility, duty and moral self-discipline and citizenship—subjects possibly suggested by the depressing reception of the Patrick Henry campaign, but also by his brother Larry’s promotion to the rank of brigadier general of the Army Reserves: Larry “did it the hard way … he took the long route, all the way from private to general officer.” Another story came to him: Starship Troopers. Later, Robert recognized the origin of the story in something his father had said when he was just five years old: “I just remembered where I got the basic thesis of S. Troopers. From my father—his conviction (1912) that only those who fought for their country were worthy to rule it.” In 1912 the country was in the middle of the militarism associated with the Progressive movement—a time and place in the culture that persisted all through Heinlein’s own upbringing, and of which the Civilian Military Training Corps Heinlein attended while he was in high school was a part. It was undoubtedly that connection that suggested a young man undergoing military training…

“I do not know that this system would result in a better government—nor do I know of any way to insure “knowledgeable” and “intelligent” voting. But I venture to guess that this fictional system would not produce results any worse than those of our present system. Not that I think it is even remotely likely that we would ever adopt such a system.”

Moreover, from the evidence that this was the story Heinlein did write and did complete, it must have had that ring of “relevance,” of engagement, he had been missing with the Podkayne story. He later explained some of his thinking in generating the story to colleague Theodore Sturgeon:

“ … I’ll state explicitly the theme of Starship Troopers: it is an inquiry into why men fight, investigated as a moral problem.… being a novelist, I tried to analyze it as a novelist. Why do men fight? What is the nature of force and violence, can it be morally used, and, if so, under what circumstances?.… What I tried to do … was to find, by observation, a fundamental basis for human behavior—and I decided that the only basis which did not call for unproved assumptions was the question of survival vs. non-survival in the widest possible sense—i.e., I defined “moral” behavior as being survival behavior … [sic] for the individual, for the family, the tribe, the nation, the race. Now this thesis may or may not be true, but it is the theme of the book, explicitly stated over and over again—and every part, every incident, in the story merely explores some corollary or consequence of the basic theorem. Is conscription permissible morally? No, because moral decisions cannot be determined by law, by committee, by group—to fight or not to fight is a personal, moral decision. Everything in the book turns on this single theorem…“

He framed his ideas slightly differently for Alice Dalgliesh (juvenile editor at Scribner’s who rejected Starship Troopers):

“Let me state the theme of my story: the central theme of the story is John XV 13: “Greater love hath no man than this, that a man lay down his life for his friends.” The story starts with a boy, a child, a spoiled son of the extreme right, one who is utterly incapable of conceiving this ideal. The story ends when he is perhaps only two or three years older, but fully matured, a lined and tempered adult wholly dedicated to that simple, selfless proposition.”

Robert A. Heinlein: In Dialogue with His Century, Volume 2: The Man Who Learned Better (1948-1988) by William H. Patterson Jr.