If we have to invent an enemy alien civilization can I suggest we decide to go to war with the Culture?

Think of it: godless, drug-using communists in space!!!

http://en.wikipedia.org/wiki/The_Culture

[NB I've always suspected that Iain M. Banks is a Special Circumstances agent - installing an author to write "fiction" about themselves is exactly the kind of sneaky thing the Culture would do]. ;-)

And if they do exist, which of course they don't, quite a lot of people might just decide they rather like the idea of the Culture and immediately defect/surrender.

I know what I would do given the choice!

Traitor!

If it means I get to belong to the Culture, yes!

There doesn't seem to be much reason to go to war with them though - wouldn't it be more likely that the entire earth would unite to convince them to let us join?

I think hostile aliens are a specific example of a more general unifier: non-human common enemies. This might include hostile AI (think SkyNet), solar radiation, or perhaps even a terrestrial worldwide pandemic (bacterial or viral).

I think that enemies you can see and anthropomorphise are more powerful, however, so I imagine a humanoid alien or terminator would be a better unifier than an invisible bacteria or global warming, even though all could be considered a common threat to humanity.

I suppose that is because of the evolutionary mechanism to keep the species alive. There's something in place that will unite us when extinction is a possibility (I believe this could be part of reason why wars eventually end..).

There's no such thing as an evolutionary mechanism that keep species alive.

I believe an extra terrestrial threat that we can visualize and also threatens whole humanity can unite us other than i doubt any worldly issues will unite anybody.

The aliens are going to get their a@@@@ kicked... in Afghanistan.

the problem is that aliens who would be able to reach Earth to threaten it will be so much more technologically advanced that any resistance will be quite futile.

what I think should unite us is the understanding that this tiny blue dot in the middle of Milky Way is the only thing that keps us alive, so we need to strive to keep it so for as long as possible while searching for other ways to survive in space on a large scale.

We both think that should happen, but for most people the need just isn't pressing enough and the scales of time are just too big to be appealing. People, generally, just respond to basic instincts (wanting to be entertained, titillated, make easy money, stuff their faces on the couch, etc) and short term, easy goals.

And unfortunately, it's unlikely to become a popular political position either.

Imagine a chance to route defense spending to space research and exploration - it's the geek dream!

I agree; if current levels of existential risk aren't enough to unite humanity, I don't think anything survivable will do so.