I once worked with a guy who fought in World War 2. He explained how secrecy was kept: General Patton (or whoever) would get the editors of the Washington Post, New York Times, AP, and others into a room and say something like: “We invade Normandy on Dec. 7, and any of you that print that will be arrested for Treason.” Never a problem back then...
For amusement (and to show consistentcy of evil), you should read the glowing articles about Hitler and Stalin from the NYT.