Poul Anderson 
Shield 

Koskinen had returned to earth with a strange new "Shield" — a device which enclosed the wearer in a force shield which absorbed all energies below a certain level. Light could come through the Shield, but no weapon known to man could penetrate it.Koskinen had developed the Shield in collaboration with the Martians. From the moment of his return to earth he was in deadly danger. His own country sent men to kill him to prevent the Shield from falling into enemy hands.Soon the whole civilised world was searching for this one man — a man armed with the greatest potential military weapon mankind had ever seen the only question was which power would possess the Shield as its very own?