Is an Alien Invasion the only scenario in which the entire world would work together?
The entire world just did work together to combat climate change at COP 21 in Paris. They met, they negotiated, and they came up with a set of goals that will hopefully slow the pace of climate change, and give us time to continue to find better ways to address it.
The brutal truth is we don't all get on, BUT, if you take a moment to look through the reports and analyses of the last Ebola virus outbreak it should give you hope.
It wasn't perfect, but I have little doubt that should the need arise, (and I don't include the 'rogue' states, e.g. North Korea IMHO), we could work together.
Potential disasters such as a pandemic are great levellers. Whether we are from advanced western nations or poor countries without major resources, we all end up the same version of dead.
Even under an alien invasion the whole world would not work together. Plenty of EA (EArthlings) would be bribed by the Humanoid Extra Terrestrials Living Among Us (HETLAU) and would become traitorous complicit EA. This is already happening the EA just can't figure it out.
It is all either misdirection or subnation (subliminal dissemination) by the Government (Hextocracy) and Military (Hextary) in preparation for ETA (Extra Terrestrial Architecture) disclosure.
If EA (EArthlings) would sign the Extraterrestrial Disclosure Petition they could force premature disclosure.
you exaggerate. In most civilized nations there is not that much emnity between neighbors, cities etc. If there were a world wide disaster looming, say a rising ocean people might be persuaded to work together to prepare for the hazard. I think it wold have to be a global threat of some kind.