I am writing a fan story for a well known universe with typical Sci Fi armies. (For those who want to knows which, in this universe people seems to love Pauldrons and gothic architecture)
They basically fight like WWII in spaaAAAAaace, and their best writers often include real life concepts and tactics from this era. Theirs space ships are a WWII navy, in space. Their air force are a WWII airforce with space jets, their tanks are space tigers. They have space marines, humongous mecha but don't like to bombard planets full of humans because it lower the number of sla ... loyal workers.
But in the canon of this universe, we often see woefully undermanned force attacking planets. Sometimes with as few as a dozen thousand men, and not super-men, to attack/control a billion+ population.
So here is my question, how strong should be one of these army to take a pretty centralised, typical Sci Fi colony with a population of 500 millions, slightly lower tech than the invader.
Let's assume that the invader can have a max of a few hundred space-marines/spartans equivalent and less than a dozen humongous mecha, and none of that for the defender.
We will also guess that this is a conquest war, not a extermination, the locals rebelled for, say, taxes and freedom of something (dirty traitors). They will seriously fight for what they want but are still within the scope of negotiations.
Edits after comments:
- The planet is WWII England ... in space ! Not everyone is in the military, but most spent 1-2 years in training. Military have a lot of initial support that will go down if the war start to linger on damage their lives too much.
- The task force carry enough supply for the first few month of combat and will be resupplied by out of system sources if the planet start producing&trading with the empire of the task force.
- The task force can receive reinforcement.
- The task force will need a few month and up to a year to reach the rebel planet after the war declaration.
- Annihilation of the planet population and production center is the last resort. As much as possible the world should keep its production capacity. So, actually, a good part of the infrastructures and population can be targeted, we only need to keep the key industries up and running, even if we have to import other resources afterward.
- The planetary government knows this and will first try to make the planet too hard to conquer. If they can hold long enough they will have more negotiation tools. (The task force will be needed elsewhere and they can ask for a better status)
- The defender have very limited access to offensive weaponry space wise. Let's say that, due to their static position, it's trivial for the attacker to dispose of them in the first few days of the assault.
- The defender have very limited access to low orbit transports. But the attacker have dropship in high number, can unload its whole capacity in a week and can move its space marines around the planet in a matter of hours.
Edit for bonus : Invasion Objective
Bloggs answer is very satisfying, partly due to sheer logic and partly because it justify the existence and usefulness of small sized space marines task forces in many universe.
So, just as a bonus, for this very universe, the core goal for the big empire is to get resources, manpower or industrial production from each of its worlds. So the goal of the taskforce would be to keep the factories running, but to manage to get their production offworld in less than 6 months. If this is accomplished, the taskforce or a part of it can stay longer to get compliance or further negotiate a reintegration of the planet in the empire.
(PS: My first guess was around a dozen million men, factoring in superior tech and a potential higher fighting ability for the invader. With all support scaled up to the task, so some scores of thousands of tanks and planes.)
NB: I now realise how wide this question is due to political, military and scifi questions, still it's something that seems really interesting to me because official writers get it wrong very, VERY regularly.
NB2 : I am editing this as an anonymous user, the StackExchange did not link this question to my account after its creation so It's still an anonymous question, sorry guys :/