Tactics and strategy inform or should inform everything a soldier does.
Your soldiers in military sci-fi must have a doctrine of some kind which tells them what to do and how to do it.
In the modern US Army, the nuts and bolts of things soldiers are trained to do are called Tactics, Techniques, and Procedures (TTP for short). The larger picture is strategy, and there are some pretty basic things that are to be accomplished with strategy.
With strategy, you must ask the question: Why are your military units doing what they’re doing?
If it does not have an applicable purpose, then your officers look incompetent, stupid, untrained, or insane. If the character is portrayed as competent, then his inability to understand strategy will be seen as a function of the author’s inability to understand strategy and the reader will stop reading the book. Do not misunderstand this to mean that officers aren’t sometimes wooden, textbook responders, or unable to remain focused on the objective. It means that if you portray a competent officer, he will respond to stimuli in a competent manner, based on the information they receive.
A competent officer has training to teach him to gain information about the objective, evaluate the enemy forces and intent, and either respond or initiate a response.
The things we will see in your novel will reflect whether your POV character is a grunt or a officer. Sometimes there’s officers who are ground-pounders– that is, company level and lower– but the higher ups formulate strategy based on the size of the units they command. A general may command an army (or divisions), and lower ranking officers command smaller sized units all the way down to the company commanders who are in the field supervising their platoons.
One important aspect of strategy (and indeed, tactics) is sustainability. That’s the dull world of logistics. Did I say dull? It’s not dull if you’re on the pointy end of the spear and you are getting no supplies, or worse, the wrong supplies. Horror stories abound from how landing ships were configured for the US Army landings in Morocco and Algeria. The doctrine of loading a ship with stuff wasn’t advanced at all, and I believe the stevedores were often left in charge of determining what would be loaded where. This led to situations where the guys first on the beach need tanks, or jeeps, or ammunition, or medical supplies, and instead they’ve got bales of blankets or rations or underwear. It sounds absurd, but the military will grind to a halt if they don’t have petrol and bullets. And water. And boots. And artillery.
Therefore, if you want to invade a planet, a space station, or another ship, you will need logistics: Transport, gas, food, drink, armor, weapons, communications, shelters, clothes, ammunition, and batteries. It’s not enough to go in with just the stuff on your back. You will need resupply if you’re not living off the land. One way to cut off an army is to interdict its supply lines, and those supply lines in space are ships dragging supplies around and factories on the ground factoring. Cut off the ships, you cut off the ability of the attacker to support an attack and you conceivably will win the engagement. We see this doctrine in seige warfare, starting over 3000 years ago (the Greeks), and continuing to the present day (Cuba).
So your logistics arm is going to acquire, store, and move stuff. It finds replacement personnel and stores them. It acquires, stores, and moves weapons systems. It provides medical, legal, and psychiatric care for personnel. If any of these things does not happen, it will affect the effectiveness of the personnel by lowering their morale and inhibiting their ability to fight.
The mindset the US Army has is that, “(1) In combat, Infantrymen who are moving are attacking. (2) Infantrymen who are not attacking are preparing to attack.” (FM 3-21.8 The Infantry Rifle Platoon and Squad, published by the United States Army.)
Overall, the strategies used in sci-fi are going to mirror those used in modern combat. Asymetrical warfare, using small unit and terror tactics, must be responded to by the larger force with a unified strategic doctrine that places forces in places to quickly react to provocation or to search for and prevent attacks before they happen.
What is the larger strategy? Follow the money. No matter how you posture your future, ultimately there are never enough resources for everyone, and that is the crux of most decisions made by the polity. It’s not for the good of the people, it’s for the good of commerce. Ultimately, no matter how deep a political system may lay in socialism, it will collapse without the application of capitalism. Therein lays the logistics argument: To get all this stuff, you must mine it, grow it, or manufacture it. The strategy is to protect the stuff, or to take the stuff.
I see some authors who promote a character from a shivering private up to general grade levels. Being a grunt does not mean you are trained to think like an officer. It means you’re trained to think like a grunt. When you have Private Schlomo promoted way above his pay grade, he’s going to be way out of his comfort zone because he isn’t trained to understand the fight at company, brigade, or division level. A fireteam leader or squad leader has 3-8 people under them, and the decision tree may not be that complex. A platoon leader, which is the lowest level of officer supervision, is the point where the thought process must be done on a give-orders-to-subordinates basis.
You must use a staff, because you are not capable of getting the information necessary to do your job on your own. At company level, you have an executive officer who seconds the commanding officer’s lead. At higher levels than company, you see more staff to assist with functions such as analysis and prediction of enemy action, communications, IT, operations to assist in personnel (promotions, moving around, exiting the service, pay, awards, and so on), and training (both finding locations and creating the criteria for what is being taught, writing manuals, creating videos for training).