In the canon of Star Trek, humanity had a series of eugenics wars, wars for resources, and then capped it off with a World War 3 where the Geneva conventions were scrapped with nuclear, chemical, and biological weapons used by every world power.

The mess was still being cleaned up and the effects of hundreds of millions dead is when people went "Why did we ever hate each other? What did some dude in the African Union do to piss off some dude in the European Alliance, that pissed off some dude in China?"

And then when Zefram Cochrane used the Phoenix to test warp flight, everyone truly went "...Holy shit, we're not alone. We can fix this for everyone."