what is the significance of the title "Zero Hour"? is it that time has ended for humanity?

Or, is it a beginning for something else? AS in 0 - 1 - 2 - 3 - 4 . . .

I always thought it was referring to the moment the attack was to be initiated. The invasion and operation had been in place for quite some time, leading up to "Zero Hour".

This story was Ray's contribution to an old anthology entitled MY BEST SCIENCE FICTION STORY. So, apparently he must think this his best SF story.

I agree. "Zero Hour" would be the time of an attack, and "Ground Zero" (as we all know) would be the place of an attack.

The episode on "Zero Hour" on the DVD with Ray Bradbury Theater is one of the best I've watched so far. Kind of a dorky ending (visually) but the story was great. The girl who played the main instigator/child was also great.