Realistically you shouldn't be able to capture anything bigger than level 6 with your marines. Even that's a stretch, Nex goes easy on you. Planning and executing a planetary invasion should by all rights be a titanic endevor. Especially since you need to bring enough troops and armaments to overpower the local garrison that can recruit locally while you can't. I would imagine any colony on inhospitable planet would be a maze of tunnels and habs that would be a nightmare to fight trough.
If we are being optimistic, the entire population of the core worlds is around 2 billion people, realistically probably much lower than that. IRL you will generally see only around 1-1.8% of the entire country's population serving as military, and only around 20–25% of them will consist of actual combat units; the rest are support personnel. And those numbers account for all military branches. Not all of the military personnel would be in the army, and I imagine in Starsector universe this distribution would be skewed towards the space force. It does seem to be the ultimate way of projecting power for all factions.
With all that in mind, conquering planets, as in overrunning established defense outposts and local governing bodies, with a fairly modest marine force, doesn't seem so unrealistic anymore. That is, if we ignore the question of how exactly the player character is conjuring up thousands of marines out of nowhere.
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