Well that seems fairly straightforward, groups form in the presence of an external threat and collapse to self interest in its absence, so if the ideal is a unified world how can that be achieved? Without overwhelming force.
The fundamental principle being applied here, is still an "Us V Them" mentality, only the "Them" is extraterrestrials.
Given how far away any inhabitable planets are, to get here to be a potential threat to organise ourselves against, would require technology far in advance of our own, and thus, they could easily win. By opposing them, we unite, and they annihilate us.
If they don't make it here, then eventually, we develop the technology to go there, and then we annihilate them for being our enemy, because we decided they were the enemy, only to get us to unite. Not exactly moral.
Anyway, it only works so long as the threat is perceived to be present. During that time, we're living under wartime conditions. Think how everyone is afraid of terrorism, and imagine that we're never free of that fear.
The rest of the time, everything goes back to divisiveness.
IMHO, not really much of a solution, more a temporary solution to a permanent problem.
fascism would work well
