XYCat: I don't know whether you already finished the game or not so i'll just say that Invisible war in particular poses rather different questions.
pigdog: I haven't finished it, no. I didn't want just to refer to DXIW but also Fallout, Zombie (virus) outbreaks, interaction with an alien species etc..
I think that a virus strain is possible that will impact a large proportion of the human race. Nuclear fallout is an obvious choice but my feeling is that it's going to be an accident in a plant rather than an act of war.
As for the other options, Zombie apocalypse is just not happening because if we rule out the occult and presume that it would be some kind of human rabies virus, then the easiest way to overcome it would be place people in quarantine and wait for the infected to just die out which would happen sooner rather than later if they only craved human flesh.
Aliens on the other hand is a rather unpredictable thing because it's mostly about their philosophy and attitude towards others on a species level. If their philosophy would be centered around them being superior beings or whatnot, like space nazis, there would be no means of convincing them otherwise because people don't usually think about what would the gang of roaches in their basement think when you unleash the exterminator on them. That would be like total war with no rules whatsoever so surival of the human race would be unlikely since the easiest way to conquer a planet would be through biological weapons that in ideal case don't harm you much, or are easy to clean up after, and wipe everyone else out, while you're sitting comfortably in your spaceship on Earth's orbit. Basically there would be no actual "war" going on if the aliens wanted to take our planet by force.