Keith Laumer 
Monitors 

Even after the set had been turned off, the TV blared the announcement:"Citizens of Earth, I am Tersh Jetterax. It is my pleasure to announce to you that a new government has taken over the conduct of all public affairs."And thus the US was in the hands of the Monitors, the strangely polite yellow-clad beings whose powers were such that they could render everyone helpless — without shedding one drop of blood. Who are they? The Russians? An alien race from another planet?