Look up at the stars. Somewhere out there someone is doing something that makes you don’t like and you want them to know just how angry it makes you. How will you do it?
In this scenario I have assumed that FTL is somewhat slow but still possible and that the end target is a developed Earth-like world.
1. Assemble your fleet.
Decide what your goals are, is this meant to be a raid against the planet or do you plan to occupy territory? Make sure you don’t leave your own world undefended when you assemble this fleet and make sure the fleet is equipped to deal with unforeseen circumstances. The ships may be gone for several years and once they leave resupply will be difficult. At least some of your ships should be equipped to harvest local resources and grow food or manufacture munitions.
2. Get Underway
Timing is important. The planet you’re going to might be years away. The main fleet should leave together so that you don’t scatter your forces too much, but it may be a good idea to send a few smaller forces ahead to scout your target. A lot can change while in transit the early you find that our the easier it will be to change course and make new plans.
You may also consider sending smaller detachments after the main force. These could be mostly resupply missions, spaced out so that the fleet can depend on resupply at scheduled points during the campaign.
Remember that depending on the technology available to you there may be a substantial period of deceleration as your approach the target system. This means your engines will be pointed towards your destination and sending out a lot of easily detectable heat and light. If this is a system that gets a lot of interstellar traffic it might be possible for all of this to be lost in noise, though at this point it would be better to assume that you have been seen and that there will be ships waiting for you when you arrive.
3. Fight Your Way Through the Solar System
Space is big, dark, and cold. Any ship is going to give off a huge amount of heat as it moved through space, especially when it starts to starts to decelerate as it approached its destination. A single ship might be overlooked, but a large fleet will be hard to miss, so don’t worry too much about the element of surprise. You can’t really hide your fleet anyway.
What you can do is make the enemy unsure of your intentions. Split your fleet into several battle groups and send them on different trajectories through the target system. This robs your enemy of a single large target and the potential for a decisive battle and allows your fleet to hit multiple targets roughly simultaneously. An advanced world probably has several populations centers through a single star system and you won’t want give your enemies a chance to attack you from behind while you’re busy landing troops on the planet.
When you do enter the system you should consider employing long range missiles. Unmanned missiles are smaller and can move much faster than manned vessels, giving them a better chance of making it through your target’s defenses. The missiles can be used to strike at targets well in advance of your fleet’s arrival to minimize the risk to your own ships.
4. Establish Orbital Superiority
By the time your ships reach the target planet their defenses will have hopefully been greatly weakened by missile strikes and their fleet will have been dispersed to deal with the multiple battle groups you’ve sent through the system. If the planet has any space elevators or sky hooks now would be the time to seize them. Ships should be positioned so that the entire planet can be observed.
5. Find Your Targets on the Ground
You probably lack the troop numbers and the will to fight for every inch on the planet’s surface. Before landing begins, select targets on the ground that might pose a threat but are also not ones you care about owning yourself. These targets should all be destroyed from orbit before any landing operations begin. Munitions used here need not be nuclear, a few metal rods dropped from orbit should be more than enough.
If multiple continents on the planet’s surface have been settled it may be worth targeting any infrastructure that might connect them in order to hinder the enemy’s ability to move their troops around. Of course this assumes that you do not want that infrastructure for yourself.
6. Deploy Ground Troops
Ground troops should only be deployed to secure targets that you have decided are vital to the success of your campaign; planets are just too big to consider anything else. Power plants, space ports, terraforming stations, and government buildings are all worthy targets as just owning them will give you control of other areas and may yield valuable intel.
The scale of the landing really depends on what your goals are. If you want to disable a few critical systems while securing intel and prisoners then you may not need to deploy more than a few thousand troops. If you plan on holding territory for any length of time then tens or even hundreds of thousands may be needed.
7. Pack Up and Leave
Annexing an entire world requires a massive commitment of resources and millions of soldiers over many years may be required to completely secure a world. If regular bombardments do not phase you then this could conceivably be accomplished with a much smaller force at the risk of rendering large segments of the surface uninhabitable.
If your goal was just to knock the system out of action to prevent them from supporting your enemy’s war effort then your options are much simpler. Once your troops have secured whatever intel or prisoners you wanted on the surface you can extract them, blowing up space ports and even power plants as you leave. Space elevators and skyhooks can be demolished by the fleet once troops have been fully evacuated.
All those space ports and elevators will likely take many years to fully replace, but to hinder outside relief then nothing quite beats a selection of orbital denial weapons (You can keep your fleet in orbit of course, but if this is part of a larger conflict then the ships are probably needed elsewhere).
A minefield in space is not going to look anything like what we see on Earth, and mines as they have been depicted in Star Trek several times would be hard to make work. Space is big and mines are tiny, you want your minefield to depend on more than just chance collisions. Here I think a swarm of small, automated satellites would come in handy.
Satellites could be deployed in order and throughout the system and outfitted with various armaments including missile launchers, rail guns, and gamma ray emitters, and if you’re more a fan of the classics then explosive satellites could be designed that would then release shrapnel to clear their orbits of enemy ships.