The invasion happened overnight (as far as the Americas were concerned, at least). The portals opened, circles of blue light no bigger than a porthole, in bedrooms and offices and stores and streets around the world, and then they closed again, just as the sensors were starting to detect them, closed again.
There were witnesses, of course; even in places where it was midnight, not everyone was sleeping, and in places where it was daylight, the portals opened in very public places. All of them told the same story:
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