The most "civilized and humane" societies today continue to employ threats, back-room dealings, assassination and torture among other things in order to secure their interests and they commit such acts against other humans. Considering that we so far on a whole have no qualms in coercing our fellow humans by whatever force we deem appropriate, why would we take such lengthy measures to ensure the fair treatment of aliens who often greet us with acts of aggression?
We have not gone through World War III, nor first contact.
The premise in Star Trek is that those events fundamentally altered our perceptions and values. They were transformative.
And that of course is where the hope comes from (IMHO).
That is a very good point, I guess i'm just the eternal skeptic when I think those events would worsen our compassion, not foster it.