I very highly suggest reading these two resources:
Much of the details of the siege will depend upon the exact objective of the aggressors. For instance, what do they need to keep intact?
- industrial infrastructure?
The more of the planet you need to keep intact, the more effort and time it's going to require to conquer.
If you're just trying to eliminate a threat, then bombard the surface with enough asteroids and/or nuclear weapons to destroy all civilization, it doesn't matter very much whether you kill the population or not since the survivors will be too busy just trying to survive to think about getting even.
Presumably your attackers want to do more than that and they want something on the surface of the planet captured relatively intact. Since the attackers don't have the troop strength to capture it directly, they're resorting to breaking the will of the population to make them give up that item.
Let's assume the attackers are coming from another solar system.
Then the advantages that the attacker has are:
- They get to choose targets
- They can concentrate their forces
- They can run away if/when needed/wanted
- They can change their strategy as needed
- They know the defender's objective (to make the attacker leave)
- They can see large concentrations of the defenders (either military,
industrial, or population on the planet surface).
- They can hold valuable features on the surface of the planet hostage
(e.g. "if you don't surrender, we'll nuke LA")
- A very long supply-line, so long that, in effect, anything lost
during the conquest you can assume can't be replaced.
- They will be completely visible to the defenders at all times (you
can't hide in space).
- Unlike normal siege operations, the attacker doesn't get resupplied.
Presumably the defenders planet is the only inhabitable one which
means the attacker might run out of food before the defender does.
Assuming the planet is inhabitable to begin with (siege of an airless body of the Moon would be totally different), the defenders have many advantages.
In a planetary siege the defenders have many advantages:
- You have access to your industrial base
- You have access to your population base
- You have more troops
- You have more resources
- You can replace your losses
- You have access to terrain in which you can hide (e.g. oceans,
- You have a tremendous ability to dump waste heat (for those super
powerful laser beams to sweep the enemy from the sky)
- You mostly should be able to feed yourself (this might actually lead
to mass starvation of the population but a suitably determined
defender might allow an excess population to starve while he kept his
- If the defenders know the attacker's objective (e.g. capture the
biosphere relatively intact for colonization), then they can hold
that objective hostage (i.e. "if we lose we're going to bombard the
surface with our remaining nuclear weapons so you don't get it
- If the attacker doesn't have enough vehicles/satellites to watch the
planet from all sides, the defenders can hide activity (ground
movement, launches, etc.) from the attacker using the bulk of the
If necessary, a war of attrition strongly favors the defenders. Exchanging almost any number of defenders to get a single attacker is probably a winning strategy for the defenders.
The ground and especially the ocean provides an awesome opportunity for mobile forces to remain hidden from view. The defenders need only use passive sensors to watch the attackers, move nuclear powered submarine nuclear missile boats into the path of the attacker's orbits and then launch missiles when they're out of view. Once the rockets finish the boost phase they become much harder to detect.
Due to the nature of the engagement, the attackers can see any concentrations of effort (industrial, population, military, etc.) and bombard them to keep the population properly subdued.
In space combat and under normal circumstances any object that isn't thrusting is at risk. If your enemy can predict where you'll be at a certain point in time, they can ensure an asteroid or similar unwelcome event happens at that point in space and time. If you are on a planet's surface, this means you.
Any fixed defensive installation that you have will get at most one shot before the attackers figure out where it is. Then they'll stay out of your firing envelope and ensure you get to see the closeup view of the underside of an asteroid.
Planetary Sieges are not Medieval Sieges writ large
Based upon the above, there will be almost no similarity between the two. The attacker is the one who will worry about the duration of the siege.
But really, read those resources. There's a bunch of information there and many people put a lot of thought into them.