Suddenly a light came on inside my brain. With the massive distances between stars it might not be economically feasible even for a very technologically advance alien society to send a huge armada to invade earth. When and if we get a visit from another civilization, I suspect it will be one ship that goes into orbit above our planet. There is a 50% possibility that the visit will be a peaceful one with no sinister ulterior motives. On the other hand there is a 50% probability that the visit will have very evil intentions. If the bad guys park in orbit above us, one of two things will happen as follows:
1) Some alien virus will be introduced that kills the whole human race rapidly. Our infrastructure and wealth would be there for the aliens to take over without firing a shot.
2) Something will happen to fry all of our electronics. All of our weapons will be neutralized.Vast social unrest will follow as people realize they have to live without electricity and all the toys that they are used to. (During the Cold War the Russians had six high-yield nuclear weapons designed to be detonated high above the US. The electromagnetic pulse that followed would fry all electronic devices and rapidly bring the US to its knees.) We would be reduced to almost a Stone Age existence and easy to control.