Impostor

After Mankind has finally abolished war, the Earth finds itself besieged by the alien 'Outspacers'. Only a protective force field about the world prevents large scale invasion-- but one scientist finds himself the supposed target of a small scale invasion when paranoid authorities are convinced he is an alien robot armed with a powerful bomb.

This story has been published previously in several story collections, and was the basis for the movie Impostor (2001) starring Gary Sinise.