Is there a word for a message that is intended to be intercepted by an adversary?
There is a kind of message in espionage that is meant to be intercepted by an adversary for the purpose of spreading false information: For instance, by sending a letter stating that troops are moving north when they're really moving west. As an example, this tactic was used by the British during the American Revolutionary War, according to the National Library for the Study of George Washington website.
British Generals Burgoyne, Clinton, and Howe, several times during the war, created letters with false information that they hoped would fall into American hands. They hoped the Americans would be deceived by the information.
I'm wondering if there is a word to describe this kind of message, colloquial or otherwise. My initial research centered around the term false flag, but that term refers specifically to disguising an activity by making it appear to have been carried out by a different party.
Q:
Is there a term of any kind that means "a message sent that is meant to be intercepted by an adversary for the purpose of spreading false information?"
An example sentence would follow from the description above:
Though the letters appeared to describe British troop movements, they were ______, giving Washington an inaccurate picture of the battle lines.
The word you are looking for is disinformation. From MW:
false information deliberately and often covertly spread (as by the planting of rumors) in order to influence public opinion or obscure the truth
I would use decoy:
a person, thing, or action that lures another into danger or a trap
The word decoy is also used in military contexts:
The decoy in war is a low-cost device intended to represent a real item of military equipment. They may be deployed in amongst their real counterparts, to fool enemy forces into attacking them and so protect the real items of equipment by diverting fire away from them.
It is worth noting that the only purpose of a decoy is to make the enemy focus their attention in the decoy to protect the real target. The word "decoy" does not imply "meant to be intercepted". If you want to emphasize "meant to be intercepted", see bait, honeypot and sting operation instead.
Therefore, in your sentence:
Though the letters appeared to describe British troop movements, they were decoys, giving Washington an inaccurate picture of the battle lines.
Red Herring comes to mind for me.
Via Lexico:
A clue or piece of information which is or is intended to be misleading or distracting.
Though not strictly limited to espionage - it meets your criteria of something intended to be seen with the goal of misleading or distracting and can easily be applied to the example.
Though the letters appeared to describe British troop movements, they were a red herring, giving Washington an inaccurate picture of the battle lines.
Here's a nice link to the etymology: Where does the phrase "red herring" come from?
Specific to the example you give, it would make sense to say the letters/missives/etc. were planted. That is:
Though the letters appeared to describe British troop movements, they were planted, giving Washington an inaccurate picture of the battle lines
You could combine this with other answers: for example, "though the letters appeared to describe British troop movements, they were in fact planted as part of a deception operation. The disinformation contained therein..."
You could say that the messages were a ruse:
In military deception, a trick of war designed to deceive the adversary, usually involving the deliberate exposure of false information to the adversary's intelligence collection system.
ruse. (n.d.) Dictionary of Military and Associated Terms. (2005). Retrieved October 1 2019 from https://www.thefreedictionary.com/ruse
In your sample sentence,
Though the letters appeared to describe British troop movements, they were a ruse, giving Washington an inaccurate picture of the battle lines.
For an example of this usage, see this question on the History Stack Exchange, discussing whether the famous Zimmermann Telegram was a ruse designed to be intercepted.