Keep Mum

Of course it makes sense, and there's still rules on all this today-- but something about the wording and graphics they used to remind you not to talk about military stuff in the 1940s is just terrific, isn't it? WPA (American) and British WW2 posters.

American. Source

American. Source

American. Source

American. Source

American. Source

British. Source

British. Source

Be careful of beautiful woman, remind the British. Source

 Though the Americans were well aware of this threat too! Source