Some unknown enemy - an invisible foe - has done this.

Small wonder that crooks had swung in the wrong direction, to meet their invisible foe!

We fight, as St Paul says, against an invisible foe.

He could see, then, as she fought an invisible foe, that it was water.

For three days, Delmuth had been playing a tricky game with an invisible foe.

Now, as then, there is an urge for covert action to combat an invisible foe.

Taking him for an invisible foe, they were trying to drop him in the darkness.

It was like a challenge to some invisible foe to come and fight bare-handed unless weapons were preferred.

He fought his invisible foes but struck only bed covers.

The two guards who had started across the courtyard stopped and raised their partisans as if to strike at an invisible foe.