An overwhelming and very clear alien invasion which we very likely would get annihilated in because we're so slow to react and likely way behind in military capabilities to a completely space faring species on conquest.

We'd get neat little merch for a while though, like Earth United stickers and fun commercial slogans with happy music showing our unity and how we will drive back the invaders , for a time lol.

Other than that, global pandemic nope, can't see it, not real enough. Maybe if it was a horrifically visible illness like blood vomit that is transferrable through direct and clear cut contact almost instantly.

Meteor is too localized and anything left standing will just tend to itself then exploit the weakness of others. Other wise it would be too much and just wipe us out to make govs unify.

Global warming would be interesting with sea level rise, may see a haves vs have nots kind of global unification in alot of ways.

Soil spoil globally into famine, would probably cause more division than unification for sure.

AI? At this point our very out of touch leadership won't even see that subversive threat coming if it does lol.

Legit, only direct and clear violence can unite us, maybe and that's likely to protect our power structures moreso than the people. Likely will still have in-faction fighting anyways on tactics, sympathizers, who is in-charge, etc.

Fortunately if all the global governments collapse the aggro that we have as individuals would probably also be why we scrap and cling to living even if we're prone to in-fighting and short-term decision making.