[FrankB]

Here in the U.S. they did shut down all ham activity. I am not sure of the dates though. I can't access my old issues of QST to find it out.

Many ham transmitters, receivers, and parts were "donated" to the war effort. the government put out a call to the hams for equipment and parts., especially panel meters.
I remember reading about one U.S. ham who, in the service, was "reunited" with his own ham transmitter where he was stationed. He was overjoyed when he found his own call on it.

The hams were not totally out of business though. They started using "Carrier Current"- (That's sending messages through the power grid wiring). I am given to understand that was the "in" thing to keep your skills up.

Carrier Current is still used here by the power companies. Very similar to the railroads communicating through the rails. (That's why every section of rail here has a jumper bonded by welding or brazing from it to the next section).