First ask yourself where they would come from and how they would get here. If you have done your homework on interstellar travel, you will find it hard to believe that any sizeable armed force could cross interstellar distances, and if they could, that they would be worth sending. (Who wants the gold and women of Earth if fetching them will take say, 1000 generations each way? The gold might still be good when it arrives, but the women would be past their best, even for dinner.)

It follows that any visitors would be in small, dedicated forces (dedicated to what? I don't know, but it could hardly be conquest!)  So we could expect them to be friendly if they are interested at all.

If we could expect anything in particular, that is to say, which frankly, I do not.

My view has always been that such a trip would only be successful if launched from a civilisation that had long-term stability, strong cooperative tendencies, and self-sacrifice as a built-in genetic standard.

Then I remember what those God-fearing Europeans did in Australasia, North and South America, and Africa, and I'm not so sure anymore. Stability and cooperation are just as likely to be based in militarism, religion and totalitarianism as in idealism, fraternalism and generosity. How fussy will mankind be when we have made the Earth uninhabitable and try to move on?

Invasion may not be necessarily by material means. About 50 years ago, Fred Hoyle wrote "A for Andromeda", an SF story where an alien civilisation sends a coded radio message that contains a design for a computer so highly advanced that the compulsion to build it is irresistible. And inside the computer is the viral payload - an algorithm to build an irresistable alien in female form who can rise to rule our world. [Excellent Wikipedia article].

Good points Paul, though they left me with a few remarks, not that I expect that to surprise you!

Militarism, religion, totalitarianism, idealism, fraternalism and generosity and similar nonmaterial values are the only ones likely to motivate interstellar travel at all, let alone on a scale calculated to support a military invasion, or even a peaceful immigration. Work it out. What would it take to achieve large scale interstellar travel? Who would pay for the logistics? Who would gain, and when?

I have long ago come to the conclusion that mankind is not in his current form adapted to any such initiative. The reason is that we are apes, and not termites. We do things for ourselves, usually, our children fairly often, our grandchildren perhaps the same, our descendants beyond a few generations, hardly at all.

Furthermore all our ancestors that are to be remembered and honoured to the end of time do pretty well and even their names are known to the general public two generations down the line. How many World War II generals can you name off the bat? How many heroes? How many patriotic wars of even the second half of the 20th century? And I do not regard you (and certainly not myself) as even typically ignorant of such matters.

Now, in termites memories of such greats probably would be even poorer, because no one would be interested. Everyone would be doing his best for everyone. Until the variously socialistic movements recognise this fact, they don't have a snowball's hope in hell of getting off the ground. None of them so far have even avoided the largely self-defeating trap of despotism.

Anyway, I digress. You ask how fussy mankind will be when we have made the Earth uninhabitable and try to move on? It hardly matters. We won't move on.

We are apes, remember?

If we meet termites coming the other way, they are the ones who will be doing the moving on. So far it simply is fortunate for us on this planet that we had our grass stalks ready.

You say: "Invasion may not be necessarily by material means." Yes. I read "A for Andromeda", an SF story that was characteristically brilliant in its lateral thinking. However, apart from certain practical details, ask yourself, assuming that we attained a level of technological competence that enabled us to achieve anything of that type, what it would take to induce us actually to do it.

Suppose that we demonstrated the existence of a suitable planet with a technologically adequate civilisation just 100 light years away, (right in our backyard). How would we go about it and what would it cost us and what would it benefit us and who on earth would it indeed benefit? These are matters of idealism. Idealism with a large pricetag and a still larger requirement of patience .

Just something to think about.

I will read the Wikipedia article that you recommend, but in turn I recommend anyone to read the early Fred Hoyle science fiction. It is deeply and widely flawed, but gives one plenty to think about.

This has been debated for decades in the Science Fiction community and has produced almost as many theories as there are participants. Unless a much faster than light form of travel is invented there are very few things that could make the time (and I assume money) needed for space travel on a large scale.

War: what could they possibly want that is worth traveling to Earth and fighting us? I think the only possible commodity is livable space. If habitable planets are very scarce, then maybe taking over another planet might be worthwhile.

Friends: again very few things could be worth the trip from a financial (or whatever motivates them) point of view. Art, ideas, philosophy. Maybe just curiosity would be enough.

But, who knows what would motivate an alien? Maybe their only driving force is to eliminate all other life forms

i think am important thing to remember is that aliens didn't sign the geneva convention it would be really easy to wipe us out and then they'd be sitting on a planet where a few thousand years of mining had put a lot of the useful minerals on the surface plus if you throw in a few plagues and so on you have a handy labour source at controllable levels to shift it about for you,all in all invasion is a viable option you just have to do something about the travel times