system
Given the following post, respond as if you're commenting on Reddit:
human
(General) Why would someone invade a planet already settled?

Why would we invade a planet? If it's settled then all you would need to do is establish a blockade and make any ship coming off planet pay a tax and if it doesn't it's boarded and taken or just blown up.

The argument against is that it's really hard to have that many ships surrounding one planet, let alone multiple systems. However when you think about it you probably need 100 million to 1 billion troops to attack, take and secure a planet.

The only reason I can think of invading a planet is for religious reasons (cleanse the infidel) or genocide (which can fall under religious).
gpt
Well for the reasons you listed (and genocide doesn't always fall under religion).

If you don't need the tax money then a blockade is pointless. Maybe the native population doesn't even have taxable space flight (earth).  Could be territorial and we're too close to their home or political and were made out to be a threat (earth peoples are terrorist!). The invaders might not be intelligent and just attack anything they can find. Maybe your space craft suck but your ground game is unmatched or you receive great honor in face to face combat?


Edit: Forgot one, maybe they need to build a highway!
Seems to me when were making justifications for war... any reason will do.