See? Right there. Why is the first thing to come to your mind physical harm against someone else? And there was no moderation about the level of harm, either. You skipped the foreplay and went straight for a nuclear assault, which currently represents the worst kind of physical damage the human race can inflict.
If the answer to violence is more violence, then you're not saving the world or trying to. You're only saving/thinking of a few, and yourself (and your ideals and beliefs). Even if the idea is noble in the end, and you could somehow justify the means to the ends, you'd end up with more conflict than you had before. Thats just a fact of life.
Now, if we could stop people from retaliating completely then this wouldn't be an issue and justice would prevail without interuption or disturbance. But for every action theres a reaction, so its all chain-linked together and it doesn't end, not even after one or both sides attain their goal. And "justice" is a matter of opinion anyways. Theres a certain level of human justice that we all try to adhere to as humans, but in war we forget about it because justice has no place in war nor is war a place to attain it. By setting out to kill people, for whatever reason, you've done an injustice to life yourself. Theres no white washing a truth like this.
It is in fact alien invasion that is the only thing, in my opinion, that would band the human race together, if for a time. We say the same for natural disasters, but is that really true? Not really. Some countries are left to die because we already know in forethought that they are inevitably lost causes, or they are deemed so early on.
Alien invasion would require our combind efforts (assuming they are more advanced than us in every way and outnumber us as individual nations). You're talking about the earth's resources, the collective military resources of the entire world being brought to bare against a single enemy. That may bolster confidence and a sense of brotherhood, but who knows if we'd fall short or not or how long that unity would last (before, during, and after the invasion).
I mean, if they have some kind of Death Star weopon, then $hits for beans, it doesn't matter what we have or do, think or say. LOL
This is actually very close to the nuclear weopon topic a while back, and I actually made this exact same point there too. Deja vu for me. 😀