'If You Talk Too Much, This Man May Die!', Fort Hood, January 1943, during WW2.
From the Library of Congress:
"Every man, a poster at Camp Hood. Taking a cue from the Office of War Information (OWI) poster "If you talk too much, this man may die",
Private Ivan A. Smith, editor of the Camp Hood Panther, Camp Hood, Texas, originated this novel method of reminding his fellow soldiers to practice discretion. Looking into the mirror at the camp".