[rrc]

I think hostile aliens are a specific example of a more general unifier: non-human common enemies. This might include hostile AI (think SkyNet), solar radiation, or perhaps even a terrestrial worldwide pandemic (bacterial or viral).

I think that enemies you can see and anthropomorphise are more powerful, however, so I imagine a humanoid alien or terminator would be a better unifier than an invisible bacteria or global warming, even though all could be considered a common threat to humanity.