It's called a Trash Can not a Trash Can't

Heinlein served in the military—during peacetime. Specifically, he served in the US Navy from 1929 to 1934, when he was discharged for health reasons. Prior to serving, he attended the Naval Academy in Annapolis starting in 1925. Taken altogether, this term of service is dominated by the process of training to become a soldier, and this experience is reflected in Starship Troopers by almost exactly half of the book being dedicated to boot camp rather than actual combat. In fact, there are precious few combat scenes in this tale of supposed "military adventure," limited to:

A routine raid at the beginning of the novel where the hero spends most of his time lobbing bombs at civilian infrastructure

A brief and confused depiction of "Operation Bughouse," an attempt by the humans to invade and capture the Bug home planet that immediately goes off the rails and ends in disaster

A detailed depiction of "Operation Royalty," in which the hero establishes a perimeter without resistance, manages troop reconnaissance, and finally goes to fight some Bugs in their tunnels only for a piece of debris to fall and knock him unconscious (the mission is over when he wakes up)

It's a strange absence of military action in a novel otherwise so committed to detailing the realistic minutiae of day-to-day military life, though a reasonable omission for an author who never saw any action himself.

However, Heinlein's personal understanding of the military from an entirely peacetime lens—studying, drilling, training—skews his ability to understand what a military actually does.

This confusion is most obvious in the depiction of the main branch of the military described in the story, the Mobile Infantry (MI for short). The MI are simultaneously framed as disposable grunts used mainly as cannon fodder and highly trained elites who perform special missions with unparalleled finesse. On the one hand, they fight in power armor, which protagonist Johnny Rico describes as intuitively easy to use, compensating for all deficiencies in strength and speed with "negative feedback," requiring only a few days of training at most to get the hang of (and that training mostly dedicated to using its radio circuits effectively), while able to pack firepower as devastating as portable atomic bombs. In Operation Bughouse, the MI is deployed en masse for (attempted) shock and awe tactics; while their mass annihilation is seen as tragic, it's also par for the course throughout the rest of the story. The low survival rate of MI troopers is constantly mentioned, any named character has a good chance of offhandedly dying in a paragraph summary at any given time, and even officers (who always go out onto the field with their men) are said to "buy it" in droves—one line mentions that officers die at a higher rate than privates, because they are always the first to make their orbital drops onto a planet.

On the other hand, despite this disposability, the MI is extremely choosy about who it lets in. Rico's boot camp starts with 2,000 hopefuls and ends with under 200; everyone else is simply sent home, deemed not good enough to be in the military at all. And this in a military branch that exclusively fights in power armor, which removes the physical limitations of elite combat. The training to become an officer is even more demanding, despite the fact that officers drop like flies and Rico claims there are never enough officers to enable a squad to operate at optimal efficiency. All of this is on top of the fact that even making it to boot camp is a struggle; the military actively uses its most mangled recruiters to dissuade as many people as possible from signing up, and once they do sign up the recruiter is liable to flunk them from joining any branch except as a guinea pig for weapons testing. This is a military that only wants the best of the best, the physical and intellectual elite, which it proceeds to train for years just so they can die a few minutes into their first Normandy Beach-like drop.

And while fighting an existential threat that is truly built on disposable fodder. As Rico explains: "Every time we killed a thousand bugs at the cost of one MI it was a net victory for the Bugs." This comparison is fundamentally illogical; if the military wants to kill Bugs, the Navy can nuke a planet from orbit and eradicate all life on it. The logistics of killing shouldn't be a factor for the MI, whose military successes as depicted in the story are performing state-sponsored terrorism (blowing up that civilian infrastructure during the raid is used as an alternative to glassing the planet because the goal is to terrorize them into defecting from the Bugs) and capturing a live specimen of a Brain Bug so the military can understand it better. Why does it matter how many Bugs are killed per MI, then? Why are the MI used like fodder when planet glassing exists?

Of course, everything in Starship Troopers is filtered through the perspective of its protagonist, Johnny Rico. And Rico is confused. He is confused about how he conceptualizes himself as a soldier. Sometimes he describes himself as a "foot slogger," sometimes as a "scalpel." Of course, Rico doesn't know anything, understand anything. He doesn't know why he's fighting. Is it to earn the right to vote in this society where only veterans can vote? He thinks so at first, then later decides he doesn't actually care about voting. Despite all his Moral Philosophy lecturers telling him that the current government is "scientifically moral" because it is founded on its members achieving self-sacrificing devotion to the highest possible cause (humanity as a whole), Rico seems to have no real interest in humanity and refers to civilians back on Earth dismissively the few times he does refer to them. His devotion is only to his unit. When he learns his mother died, "smeared" when the Bugs hit Buenos Aires, it screws him up a little. His lieutenant asks him if he wants some time off; Rico says no, preferring to "wait until the outfit all took R&R together." He continues:

I'm glad I did it that way, because if I hadn't, I wouldn't have been along when the Lieutenant bought it . . . and that would have been just too much to be borne.

He's able to bear not being there for his mother's death. The implication is that his lieutenant, his squad, is more important.

This flies in the face of the lectures on morality littered throughout the story. In the same lecture about achieving self-sacrifice for the sake of humanity, the instructor Dubois discusses the "juvenile delinquents" of the 20th century. Street gangs, he means. Dubois claims these delinquents were morally undeveloped because they had loyalty only to the unit of their gang, rather than to society, the nation, the world... yet isn't Rico himself only really loyal to his "gang," his military unit? Is this "scientifically moral" society not simply sublimating the exact "immoral" impulses of the street gang into its own disposable tools?

That contradiction points to the real reason why Heinlein's military is so nonsensical. Despite officers being hard to come by, despite the MI being chronically understaffed, officers drop first -- to boost the morale of the troops who jump after them. It's the romantic notion of the general who leads from the front, which flies in the face of even basic military strategy (it's bad, actually, when the chain of command gets disrupted and your best men die first), but which feels virtuous, which hearkens to heroic ballads of soldiers past (ballads Heinlein loves and quotes liberally throughout the story). Hence too the confusion about what the MI even is: it is whatever it needs to be to feel pride about itself at any given time, whether that be via expression of elite soldiering, when the MI is doing elite tasks with finesse, or the beautiful expression of noble sacrifice, when the MI is mowed down as fodder.

(As an aside: Despite how highly Heinlein esteems sacrificing one's life for one's country, he manages to mention as one of the positives of his ideal society that taxes are low. Sacrificing one's money for one's country, I suppose, is a bridge too far.)

Heinlein's vision of the military is one seen by a man in perpetual training, never in combat. A man who served safely between the two greatest wars humanity ever fought, who got to watch both wars from the sidelines. A military that can always be the ideal it wishes it was, righteous and noble, and never has to butt up against the reality of what it becomes when its true function is inevitably brought to bear.

Verhoeven's adaptation does not really critique fascism. There is no explicit commentary on Heinlein's society within the film, only implicit commentary via the visual connection to the Nazis. Verhoeven assumes the audience already considers fascism loathsome, and doesn't bother explaining why that is. In doing so, Verhoeven's adaptation becomes much more concerned with critiquing propaganda; the way an ideology can be implanted in someone, transforming them into a willing soldier for a cause. In doing so, Verhoeven much more directly strikes at the heart of what Starship Troopers the novel is really obsessed with. Is Heinlein's proposed society good, or even functional? Like all proposed societies, Heinlein claims it will be both good and functional because it will be run exclusively by good people (military veterans). At its base level, it's merely another shade of the theoretical "benevolent dictatorship," only Heinlein is convinced veterans must necessarily be benevolent. (As with all these proposed benevolent dictatorships, nobody ever stops to ask whether the benevolent dictator is competent too.) Heinlein has wholly bought into the narrative of the military, and prioritizes that narrative over the reality of how the military functions. Starship Troopers is, for all its dry logistical minutiae, simply a story where the man Heinlein wishes he was stands on top.

Before Starship Troopers, I read a novel by Louis-Ferdinand Céline, an author who was actually a fascist, or at least a Nazi collaborator in occupied France. This novel, the semi-autobiographical Journey to the End of the Night, details Céline's experience in World War I, a war Heinlein only ever understood from a distance. Céline recounts being caught up in a sudden rush of patriotic fervor to enlist, only to realize in the trenches the utter horror of war, the depravity and the insanity of it, and to immediately do anything in his power to escape it. Despite this, he was wounded—wounded heroically even—and received the medaille militaire, an award that would later save him from serious prosecution after his much less patriotic activities during WWII.

Céline looked upon the medal as a joke, the war a joke, all of it a joke. He would despise war thereafter, he would despise governments, he would despise all authority. In his cynical writings he skews anarchist, if not merely nihilist. Yet he wound up carrying water for the Nazis, producing propaganda for them that even the Nazis at times found too extreme to print. Why? The reason is simple.

Céline really, really, really hated Jews.

These two novels by two supposed fascists could not be more dissimilar. Celine and Heinlein would despise each other, are in many regards utterly ideologically opposed. I suppose you can discover in this contradiction the mercurial nature of fascism, which seems to be any kind of radical right-wingism at any time. Heinlein reveals that his protagonist Johnny Rico is Filipino on almost the final page of the book, as though it's a twist, similar to the twist Verhoeven pulls when NPH marches into frame in full Gestapo regalia during the film's climax. Aha, Verhoeven seems to say, it was fascist all along—and you didn't realize! Aha, Heinlein seems to say, it wasn't fascist (or at least racist) all along—and you didn't realize!