As we train endless bandwidth on the search for extra terrestrial life
... and quietly hope we wont need to listen, because they'll all just show up and give us warp drive,
I propose that any dramatic and conclusive contact would pretty much spell the end of the world.
I'm basing my opinion on a few assumptions;
~ you don't "come to earth" from a distant galaxy and ultimately be less cool than us. Who ever they are, they're gonna out-everything us, except they'll prol'y be ugly. Bigger guns, better geek, more powerful "power" ... a whole lot cooler.
~ "they" will not be the size of gnats and be swallowed by a barking dog (tips hat to Douglas).
~ we're gonna have something "they" want. Maybe it's only our awe (which wont be hard to win) but it'll be more complicated... by a long shot.
So THEY show up (and they're not clams or zenu), buzz a few national capitals, and come 'round for tea.
I mean, besides the likely response of what has surrounded us for decades... the total xenophobic governance of aggression. "You're different... we hate you ... and we kill you dead!" Argue if you want, but you only need to run down the list of wars-in-the-20th century and that pretty much sums it up.
But besides that...
The world is going to realize that, in an instant, the biggest kid on the block will appear to have just had his stick taken away.
Planetary leadership... will be a brawl from the get go to the very end of days ... and the big players will rightly attend to the needs of planning the new order of things.
Meanwhile, all the half-way-there players... especially the ones that are only holding back all out civil or neighborly war because of the US stick... will be free to kick the crap out of each other.
And they will.
It'll be bad.
It'll be messy.
Marshal law, and a general lack of patience for the struggle to co-exist on our lonely planet will rule, because it'll suddenly be much more important to co-exist in a universe.