Among science fiction stories with space flight, the overwhelming majority are about combat, both between spacecraft and between futuristic ground troops. Not to mention the occasional starship marine assault trying to board a hostile ship while in flight. Yes, there are a few non-combat stories, mostly about exploration, but space combat is here to stay.
In this section is information about military theory and organization in general, even as it pertains to the present day. The next section focuses on the branches of the military as they may operate in a science-fictional rocketpunk future.
Author Jerry Pournelle often says that "the purpose of the army is to break things and kill people". This is true but more from the effect standpoint, not the cause standpoint.
My USAF vet father taught me that in the United States Armed Forces the first principles of war is The ultimate military purpose of war is the destruction of the enemy's ability to fight and will to fight.
So soldiers are trained to have the skill sets needed to perform their military's principles of war. Civilians may look down their noses at soldiers, but without the soldiers the civilians would be either dead or enslaved.
Soldiers Are Not Police
Keeping in mind the purpose of the army re: things and people will tell you why soldiers make terrible police, and why militarizing the police is a very bad idea. At least in a democratic society, at any rate. The skill sets of soldiers and police are totally different.
Militarized police are only useful for authoritarian governments oppressing the citizens. Such governments find it very handly to have police who are skilled at shooting people and blowing up buildings.
Turning a new recruit into an actual functioning soldier is very tricky. It ain't easy training rational human beings to advance into harms way on command when the logical self-preserving option is to run away.
In "boot camps", civilians joining the military are trained to become soldiers by the sergeants ("basic training" or "recruit training"). This includes a necessary transformation of the civilian's personality. It is sort of like brain-washing, but in a good way. A civilian mind-set will not work at all in a military environment (indeed, in a combat situation it can be fatal), the change over to a military mind-set is vital. It takes a military person to obey a command to charge into an enemy position blazing at you with a hail of bullets, most civilians would flee.
When a new soldier gets their first three-day pass to go visit home, they are often warned that "home" will seem to have radically changed (actually it is the soldier who changed). The soldier will find that their family will understand their words but not their language. That's when the soldier realizes they are not civilians any more.
After the recruit has finished basic training they then are given Advanced Individual Training. This is where they actually learn the technical skills required for their job. Basic training is mostly intended to transform a civilian into a soldier (granted they are given training in military universal fundamental skills, like how to use their combat weapon).
On spacecraft in general, and military spacecraft in particular, there will be a strict "chain of command." People with no management or military experience may not see the point behind a chain of command, but if such a person is suddenly given the task of managing a project (even a high-school bake sale) they will suddenly discover why it is vital. Attempting to run a spacecraft by a democracy or other laissez-faire system will probably result in the destruction of the spacecraft and the death of all the crew. Space is a far too deadly environment and spacecraft are far too full of dangerous equipment to leave things to chance.
The organization of an astromilitary is a complicated subject.
The structure of any military may seem a bit strange to civilians who have had no contact with it. One thing to keep in mind is that, strange as it may appear, the bottom line is that it works. You are looking at the result of thousands of years of practical refinement. Any historical armies that used military structures that did not work would have been eventually defeated, and the armies destroyed. All currently existing armies are the descendants of trial-by-combat, where they bury the people in second place.
For those who are unfamiliar with the theory behind military ranks herein follows a simplistic primer. If you are familiar you can skip to the next section. Please note that this is simplistic, and I the author am a civilian. Take what I say with a grain of salt. If you want true accuracy I'd advise you to talk to real military personnel.
The main division is between Officers and Enlisted Men. Basically the officers tell the enlisted men what to do. Specifically the officers hold "command" over the enlisted men. They also hold command over officers who are lower in the chain of command. The officers plan strategy and tactics to win the current war, and the enlisted men actually fight the battles according to the plan.
It is vaguely similar to the difference between management and labor in a corporation, but with differences.
The various ranks of officers have different names, but in English speaking nations it runs broadly in order of rank Marshal, General, Colonel, Major, Captain, Lieutenant, and Officer Cadet. Enlisted men in the US are Corporals and Privates.
In terms of the movie Metropolis the officers are the "head" and the enlisted men are the "hands."
In stereotypical formula science fiction writing, characters who are officers are often archetypes from the trope "Command Roster", while characters who are enlisted men are often archetypes from the trope "The Squad".
In between the officers and the enlisted men are the Non-commissioned officers (NCOs), though they are technically part of the enlisted men. These are the Sergeant majors and Sergeants. Officers hold command over the enlisted men, but the NCOs have "control" or "charge" over the enlisted men. Simplistically the officers command What while the NCOs control How. For instance, the Colonel will tell the Sergeant major that hill Whiskey-Tango has to be captured, and the Sergeant major will issue orders to the enlisted men to ensure that the ensuing battle achieves the objective. Sergeants are considered to be the backbone of the military.
Sergeants also have the vital role of training civilizan recruits into enlisted men, "turning boys into men". See "boot camp" above.
The senior sergeants also have the role of subtly training junior officers. As Wikipedia puts it, the senior sergeants give advice and guidance to junior officers, who begin their careers in a position of authority but generally lack practical experience. Meaning the junior officers are clueless newbies, often full of ivory-tower but totally impractical ideas of how to run things. The senior sergeants gently show the junior officers the error of their ways and show them what actually works in the real world.
Wise junior officers rely heavily upon their sergeants. It is also a serious mistake for a junior officer to make a sergeant major angry at them. There are millions of subtle and deniable ways a sergeant major can make a junior officer's life miserable.
Sergeants are the interface between the officers and the enlisted men. It is a big mistake for an officer to attempt to by-pass the sergeants and directly order the enlisted men. And if an officer wants to know the latest military rumors and scuttlebutt, they should just ask a sergeant.
There are also Warrant Officers. While they are considered to be officers, their main job is being technical experts in various specialized fields.
Officers are further divided into line officers and staff officers. The difference between line and staff is that line has the job of doing the core work of a military (i.e, winning battles) while staff has the job of administrative, operational and logistical needs of its unit.
In the novel Starship Troopers, Robert Heinlein creates a rank known as "Sky Marshal". This is the supreme commander over both the space army and the space navy. Candidates for Sky Marshal need experience in both branches. As the protagonist of the novel puts it: "A man can't buck for Sky Marshal unless he has commanded both a regiment and a capital ship — go through M. I. (army boot camp) and take his lumps and then become a Naval officer, or first become an astrogator-pilot (naval officer) and follow it with Camp Currie (army boot camp), etc. I'll listen respectfully to any man who has done both."
Traditionally, the areas of the craft closest to the control rooms is known as "officer country", while the greasy cabled and be-piped areas inhabited by sergeants and enlisted men is known as "below decks" (though which is below what becomes an open question in microgravity).
This is always a delicate question in any nation that has a military. Unless the nation is ruled by a military junta, the military is theoretically controlled by the civilian government. But how does the civilian government enforce this when the military is the one with all the guns? Or worse, what if the military decides it deserves to control or even become the government, and tries a Coup d'état? As the Roman poet Juvenal said "Quis custodiet ipsos custodes?" or "Who will guard the guards themselves?"
- If the military seized power over the government, the coup is successful and the government becomes a military junta.
- If the military is defeated the coup is unsuccessful, and the punishment for the coup leaders is likely to be very savage. Civilian politicians have absolutely no sense of humor about somebody trying to take away their power.
- If the military coup is a draw, you probably have the start of a horrific decades-long civil war.
There are a few methods of asserting civilian control over the military. They work pretty well, but none are fool proof.
Examples of military coups in science fiction include Mobile Suit Gundam ZZ, the movie Akira, new Battlestar Galactica, the story arc of Babylon 5 with the virtuous General Hague leading an attempted coup against evil President Clark, and in Deep Space 9 Starfleet Admiral Leyton attempts a coup on Earth.
Amusingly enough, General Hague and Admiral Leyton were played by the same actor: Robert Foxworth. The creator of Babylon 5, J. Michael Stracynszki, was annoyed at the producers of Star Trek. It seems he approached them when trying to get Babylon 5 produced and showed them the concept and story arc. The Trek producers turned him down. Then a few months later they suddenly created Deep Space 9, which has a suspicious number of similarities to B5's concept and story arc.
So Mr. Foxworth taking an acting job with the Star Trek producers was an act of treason, as far as Mr. Stracynszki was concerned. Mr. Foxworth discovered that he had no future role at Babylon 5 since his character was abruptly killed off in a hasty re-write of the current B5 episode.