Long before television and the internet, radio was the most powerful mass communication technology ever created. Both sides in WW2 understood this, and both invested heavily in radio propaganda aimed directly at enemy soldiers and civilians. The results were instructive about human nature: propaganda is most effective when it contains a kernel of truth, and least effective when it is obviously absurd.
The most famous Axis broadcaster in the English-speaking world was William Joyce, an Irish-American fascist who broadcast for Germany under the nickname "Lord Haw-Haw" — a mocking nickname coined by a Daily Express radio critic who found his aristocratic drawl ridiculous. Joyce's broadcasts were detailed, often accurate about British military movements, and specifically designed to make British listeners feel the war was lost.
The British government was worried enough about his effect to conduct surveys. The results were unexpected: while millions listened to Lord Haw-Haw, almost none were demoralized by him. Most found him entertaining. Many listened to laugh at his more obvious mistakes. The BBC concluded that his programs were, if anything, good for morale — they gave people something to ridicule.
In the Pacific, the Japanese deployed "Zero Hour," a program hosted by a rotating roster of English-speaking women collectively nicknamed "Tokyo Rose" by American forces. The women played American popular music and read news designed to make soldiers think their wives were being unfaithful and that the war was unwinnable. Again, surveys showed soldiers mostly found it entertaining.
One broadcaster, Iva Toguri D'Aquino, was wrongly singled out as the principal "Tokyo Rose" after the war and legally persecuted for it. Research eventually showed she had actively tried to sabotage the propaganda she was forced to broadcast. She was pardoned by President Ford in 1977.
The British ran their own black propaganda operation through the Political Warfare Executive. Sefton Delmer created Soldatensender Calais — "Soldiers Radio Calais" — which pretended to be a German military station. Delmer's genius was to make the content entertaining and the false information specific enough to be credible. Rather than crude demoralizing messages, his station reported exact details of officers conducting affairs and corrupt officials profiting from the war. The specificity made it feel real. German commanders were never quite sure whether their troops were hearing actual stories or Allied fabrications.