Oh, I do realise the unlikelihood! Unless FTL transport is possible, it's unlikely ever to happen.
I was just wanting as an example something that could not be disputed, as in a 5km long starship dropping in on the UN, as a jump-of point for how deep cognotive dissonance can go!
I don't think, if they came, they'd hand out weapons. In fact, I think any alien race that contacted us before we had socailly evolved another few centuries if that.
I imagine there being more unrest in the religious communties that would find it hard to incorporate aliens into their worldview. Many traditional Christians and Muslims would be traumatised. Buddists and Hindus would find it far easier to accept; no clash. I do think there would be a lot of doctrinal changes though, with more explict understanding of the fact that god can't be proved, but that this doesn't matter provided you don't take things too literally and realise that in the absence of a single way, it must be the trying that is important.
I suppose if SETI ever did find some thing, then there would be a great schism. The real fundies (of all religious ways) would just not believe it, ever, not even in, say, 2134 when full (slow, 66 ly each way) two way communication is established and a cultural and technological exchange programe starts. It would be weird if it were in the immediate neigbourhood though... it would be the biggest news story of all time, there would be a massive conflab about how to reply, and eventually someone would, even if it were unofficial, and then there would be two life-times wait for a reply, during which it would be a scientific curiosity and not much else.
I think it's one of those things, like asteroid impacts, that have a probability of 1 over time, but in an individual lifetime is so astonishingly unlikely as to have a probability of zero.
People living in glass paradigms shouldn't throw stones...