When aliens attack

The other day I caught a National Geographic channel program entitled "When Aliens Attack".  Basically it speculates what strategies humanity would utilize to fend off an invasion of Earth. 
Basic premise is that the aliens would be nomadic and would want to harvest the Earth for resources such as fuel and water.  But I was thinking that if aliens would travel across the galaxy to extract resources I would think they would be sending advance scout probes first to find out which planets have the resources they want.  They could also scout beforehand our vulnerabilities.  I just read a novel One Second After which is an account of what would happen after a high altitude nuclear EMP happens.  SCARY stuff!  As our society is so dependent on electrical and electronic machines, it would have devastating effects.  In the novel, it would basically mean a 80 to 90% die-off in the developed economies.  So coming back to alien invasion, if they had figured out that is our vulnerability, I would also think they would have a year or two to wait for the after effects of such an attack.  I would also suppose a space faring race would easily have nuclear weapons technology at their disposal.  That is of course if their intention is to take our resources and not subjugate or enslave humanity (or use humanity as food).  The latter would require another strategy altogether.