I am trying to make a space based strategy game, in which different factions try to take over all the planets in a solar system. The first step anyone should take to take over a planet would be to destroy any spaceships (and surface-to-space defense installations) defending it, after this, an attacker may choose to destroy the planet from space (making it uninhabitable), negotiate a surrender, lay siege or invade the planet. While I do have some idea as to what the first options would involve, I am not sure what exactly an invasion would look like.
My question is therefor: What would a full-scale invasion of a planet, by a technologically equal civilization, involve?
Based on these assumptions:
Invading a planet involves putting your troops on the ground, (which is the difference between invading a planet, and negotiating a surrender or destroying the planet from space).
The invaders want the planet as intact as possible, which includes both its infrastructure, ecosystems (if any) and the civilian population. The player may be allowed to choose how great length their troops shall go to to protect the planet, but certainly invading a planet should leave it better off than the Drop-a-few-thousand-antimatter-bombs-from-space-option
Even though the societies depicted in my game will be (significantly) more advanced than our society, their infrastructure would still look mostly like ours: They may have fusion reactors, quantum computers, hyperloops, space elevators, mass drivers and superconductors, but they will still have a physical energy, communication, and transportation grid.
In my game magnetic shields may be used on some planets to protect the inhabitants from solar radiation, but these shields are not powerful enough to provide any protection against the weapons of this time.
There is no teleportation technology, therefore the invaders can't just beam their troops down, and the defenders can't use a stargate to evacuate their planet or get help.
The weapons depicted in my game will mostly be more advanced versions of modern weapons: There may be antimatter bombs, antimatter propelled surface-to-space missiles, super-advanced stealth technology and surface-to-space artillery (possibly mass drivers), but there are no FTL torpedoes (in fact no FTL at all), no lightsabers, no hand phaser, no hand lasers, no death star, no gravitational-wave weapons, no photon torpedoes, no invisible elite soldiers and absolutely no magic.
(in response to comment) I assume that the attacking ships are very heavy ships build in orbit, which properly can't survive entering the atmosphere of the planet they are attacking, and won't try to land on it (since it will be unlikely that they will get off the planet again) The attackers will, however, have carriers which can send smaller lander vehicles which can take troops to the surface (and back).
(In response to comment) One of the main reasons for asking this question was that I wasn't sure when an invasion should be considered a success, but at this point I think the invasion would be successful as soon as all centrally organised resistance either surrendered or was destroyed, though when this happens on an entire planet can be extremely hard to tell, and judging by modern invasions of countries, fighting hardly ever ends when the capital is captured – just look at Afghanistan.
(In response to multiple answers and comments) While life-bearing planets probably will be self-sufficient in food, I think that all planets to some extent will depend on imported goods. At least because some resources aren't available on all planets and moons – of which Helium-3 probably is the most important resource.
Even though only one answer could be accepted, i have tried to take inspiration from all suggestions, thanks.