Thanks for taking the time to reply.
Geoff the Medio wrote:
To clarify, you are clicking the invade button on the planet, not just putting ships into a system?
Yes indeed. I finally managed to win one when I sent a crushing 50 troopships in.
Geoff the Medio wrote:
Have you brought in armed ships (not troop ships) to first clear out all the enemy ships in the system?
Yes

Geoff the Medio wrote:
You mention size 18 planets; what size is this? If it's population, that doesn't matter for ground invasion purposes; you need to have enough ground troops to be larger than the amount of troops on the planet (see attached image).
Yes, I meant the population. I wasn't even aware of the ground troops stat until you pointed it out. Thank you, that's a very great help!
Geoff the Medio wrote:
Sitreps that say "A battle occurred at %system%." are about space combat, not ground combat. A ground combat system should say "A ground battle occurred on %planet%." (although I'm not sure what version that message was added in). There are presently no more detailed message about the results of ground combat, though.
I must have been misremembering or something, because I see in my sitrep it DOES say "ground battle".
Thanks again for taking the time to reply. I think the problem was that I was unaware of the "troops" stat.