How would you invade a planet?

[Mateus]

Originally Posted by Figleaf23

Here's an idea.

Send a big rock from the sky to really give a good hard smash to civilization.

Then miraculously show up posing as the interstellar Red Cross. Use wealth, influence, secret force, and an aura of wisdom and helpfulness to control things from your Green Zone, while in reality setting the conditions for endless internecine conflict.

Hahaha! This is EVIL man! Excelent idea.

[Dustin]

If you want to keep the ecosystem intact, you either need to kill all the humans in one go with a tailored fast-acting plague, or you need a mostly peaceful method (co-opt and colonize).

Any genocidal method that isn't fast-acting is likely to result in massive refugee populations that will strip the ecosystem bare.

[Phoenix_Dragon]

Originally Posted by Dustin

Any genocidal method that isn't fast-acting is likely to result in massive refugee populations that will strip the ecosystem bare.

The bombardment method would still be fairly effective. Between bombarding larger population centers and destroying the primary transportation infrastructure (Mainly rail-yards, but also sea- and airports), you're going to have a large amount of the population die very quickly, while destroying any ability to organize refugee efforts. You'll get some ecosystem damage from the bombardment, and some localized damage around those population centers where the survivors of the immediate impact spread out (Although if you've wiped out their ability to militarily strike back, you should probably be hitting any refugee groups with smaller-scale military attacks). You don't even need to touch the rural areas yet; not only will they be sparse and unorganized (Easy prey for a modern/near-future combat force that's a-okay with genocide), a fair amount of them are going to be disrupted by the wave of survivors from organized areas.

Then you can move in and mop up any survivors. Keep some surveillance ships in orbit to look for concentrations of thermal or electromagnetic emissions that show any attempts to organize, and wipe them out. Even worst-case, you've got vast swaths of the planet with minimal ecosystem damage, like much of South America and Africa, much of which may only be affected by atmospheric dust from the bombardment and follow-up firestorms. Atmospheric effects will be minimal after a short period (A few years, at most), and even the worst areas of ecological damage should mostly recover within a few decades.

[robkelk]

Originally Posted by Dangerious P. Cats

... The reason for the invasion is that the planet has breathable atmosphere and compatible Eco-systems that your superiors want mostly in tact. ...

Originally Posted by Phoenix_Dragon

The bombardment method would still be fairly effective. Between bombarding larger population centers and destroying the primary transportation infrastructure (Mainly rail-yards, but also sea- and airports), you're going to have a large amount of the population die very quickly, while destroying any ability to organize refugee efforts. ...

I do believe that killing off a large fraction of a species does not leave the ecosystem mostly intact.

[Ulzgoroth]

Originally Posted by robkelk

I do believe that killing off a large fraction of a species does not leave the ecosystem mostly intact.

One species? If you're not allowed to have even one species go extinct, I recommend slapping a stasis field on the place.

[vicky_molokh]

Time to break out Mass Combat. What are the invaders' assets?

[The Colonel]

A heavy EMP bombardment would also be quite effective in destroying civilisation at TL7+ without infliciting much/any environmental damage (depending on how you generate the EMP). Burning out the power grid and most active semiconductors should be a good start.

[Phoenix_Dragon]

Originally Posted by robkelk

I do believe that killing off a large fraction of a species does not leave the ecosystem mostly intact.

I think it's fairly safe to assume that when someone who is willing to launch an unprovoked invasion of another sapient species' homeworld states the sum total of their interests to be a breathable atmosphere and "compatible" ecosystem, they don't care at all about what happens to the sapient species they're stealing it from. Just that they can move in and live there once they're gone.

[adm]

How hard would it be for a TL10 space capable species to trigger any of the various super volcanoes? Set off Yellowstone, Iceland, and perhaps have a major tsunami off of eastern Asia and our civilization has taken some major damage. An asteroid swarm careful managed should cover that, and then you can either rescue, "rescue", or shoot the survivors.

[Icelander]

Originally Posted by robkelk

I do believe that killing off a large fraction of a species does not leave the ecosystem mostly intact.

I don't quite follow. At least 99.9% of all species that have existed on Earth are now extinct and the ecosystem was certainly mostly intact after most of them went the way of the Dodo.

A Permian-Triassic level event, killing 90% of all then-living species, would play merry havoc with the ecosystem, however. Even then, it didn't change enough so that an alien species which could survive here before it wouldn't be able to after it. Of course, since the event lasted for quite a while, 'after it' means more time than the aliens may have.

But just killing (probably most of, not all) one species, even if it is a numerous one like humans, wouldn't damage the ecosystem nearly enough to matter to the aliens. Not if they can travel the stars, then they can certainly survive a few weeks of waiting while the worst of the chaos of the die-off plays itself out.