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?