Since a full-scale invasion is nigh on impossible unless you're willing to take a scorched earth policy (in addition to not minding how many thousands of your own soldiers die in the process), the specifics of option 1 are made impossible. Bombardment depends on just how advanced the weaponry that we're talking about is. Our own level of technology isn't really compatible to the idea of warring planets, but if our weapon's technology could be said to be not much higher, or for some reason not advanced as quickly as spaceflight, then I suppose that's reasonable.
As currently, our only type of weapon that would be effective in bombardment is nuclear, there is a problem. The likes of anti-matter would be much more planet-friendly, as they are clean (but devastating) weapons. A nuclear bombardment, even a single strategic nuclear missile, could cause more harm to the planet that you are willing to accept. Quite often, you'll be wanting to take the planet as your own, either for strategic value, loot or resources, so turning it into a radioactive hell (or, come to think of it, a sphere of molten rock) isn't going to be the best idea.
Sieges are also very difficult to use effectively. Although you said there is no cavalry, the chances are that if you are the type of species to be in one war, there may very well be another war or two coming along soon, with other species, and you cannot afford to keep ships tied up monitoring the planet. And of course, planets will very often be self-sufficient. Fortresses have been known to last out years in a siege, and a fortress without its distrada is hardly self-sufficient.
The only option that I believe would be successful is covert operations and lightning fast raids. Use your spacecraft to clear out aircraft and wipe out as much of the SAM installations as possible... particularly paying attention to installations in areas that you aren't interested in. Follow up by striking as many SAM installations in your target area as quickly as possible, and then landing a strike force (preferably as well trained as possible - setting up satellites and a GPS with IFF decoding per soldier could be handy.) The aim of the strike force should be to kill civilians as much as possible. Slaughter them, and perhaps pile them up if time permits. Give the rest of the world something to really shake them. Taking away large numbers of women could be useful, also. Have them killed once you get them back to the ships, of course.
Repeat as much as possible. As long as you are able to destroy SAM installations quickly, and wipe out their air craft, there shouldn't be too much of a problem with this.
Meanwhile, work on assassinations. Political and military leaders, anyone that is known to the entire world or much of it. It might be a little more difficult than the strike attacks, but assassinations do work when handled right. If the world sees pockets of it being overwhelmed and slaughtered within less than a two hour period, and all its leaders slowly dropping like flies, the people will start to get a little distressed. The government(s) will likely need to make examples to keep control, or they'll lose control right away. You'll have caused dissent and despair, and the planet should be ripe for the picking.
Alternative: Assuming you have SDI Defence
Use a slightly heavier strike team to attack one particular target. Set up SDI defences as quickly as possible, and establish a perimeter. Bombardment with the heaviest clean weapons you have around your perimeter will establish an area difficult for infantry to pass through. Hopefully, you'll have the artillery to deal with anything.
Take it step by step. SDI defence will protect you from missiles, providing its good enough. You might lose your first few footholds to massed bombardments, but hey, its expendable. You're only putting a relatively small amount of resources in one spot, and they're likely to throw everything they have at it.
This does rather rely on ground superiority. If you don't have it, it won't work.
Alternative 2: Divide & Conquer
Probably the easiest. Once you've got things going, you don't need to put much effort into it. Stir up dissent, either between governments, or creating rebellions. They'll always be people who want to surrender to you, especially if you first target areas that are known to be disapproved of by other areas. Once you've got a planetary war going, or at least the planet concentrating inwards on itself, it's time to work on taking over, although if the war is particularly bloody, it could be worthwhile to wait until its over, so they kill as many of each other as is possible.
Alternative 3: Don't need the planet
Several thousand nuclear missiles, fired all across the planet. 'nuff said. If you happen to have some technology to protect against radiation, or clean it, you can still make use of natural resources on the planet, although if you had any plans to loot it, you'll have to forget it.
Alternative 4: They surrender easily
...just kill them. If they surrender, that isn't game over, ended in victory. That means you have an advantage -- the planet isn't yours yet. Once they surrender, set up positions everywhere, and once the time is right, purge the planet. They'll regret surrendering to an evil, cunning bastard such as you.
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Originally posted by GWG:
In thoery yes. that was my 1st though but consider this. In the past there was no real international trade. Countries were self sufficient. Then there was trade. Now there are large numbers of countries who are net importers of food which their population needs to survive. If there was a viable interstellar trade system then it is not unreasonable to suppose that planets are no longer self-sufficient and therefore that blockades are possible. After all, how many countries in the here and now do not import something?
Except that they don't have any other planets, or any other allies. As Phil said in the opening post, there is no cavalry coming. Either that means you've dealt with the others, which will likely have taken a lot of time (and if you've had other planets to play with, you'll have already turned one into a radioactive hell and therefore everyone will be happy to surrender already), or there are none. I think it's fair to assume that this planet is self-sufficient, and has no outside contact other than the war with you.
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-Esponer
(This message has been edited by SilverDragon (edited 11-15-2002).)