I am adding an alternative scenario about an alien invasion, but this time is more challenging using aliens to conquer Earth. Now there is only one human civilization (which I called "United Earth") which has much more cities at disposal than the alien civilization. Aliens now are scattered in different places of the planet (and not concentrated in North America) that get their cities more isolated and difficult to defend.
I have taken the inspiration from the movie "Indipendence Day" in which aliens attacked several cities in all continents (not just North America).
Alien Invasion more challenging (using aliens) 2016-10-05
Alien Invasion more challenging (using aliens)