Have you met us - the collective us that is Humanity?!?!? I think it would be a colossal SS at a minimum!
To answer the question, I'm not honestly sure. I think that if I knew 'they' were coming and 'they' were of obvious superior intelligence and bent on destruction I'd probably keep my mouth shut and let people live their now-shortened lives in ignorance vs. mayhem and whatnot. With that being said, I'd presume SOMEONE else would know and word would get out no matter what my (as leader of my Country) decision to do would be.
Personally, I agree with whomever up thread said we were like ants. IF there is intelligent life out there I figure we are so insignificant to them they wouldn't bother with us but if they did and they wanted what we had, we'd have no chance.