Term for dilemma regarding knowledge of information which cannot be used without alerting the other party
Solution 1:
The term "Coventry dilemma" has been used in this context (e.g. here), after the supposed decision by Churchill in the Blitz not to give the city of Coventry any extra defences even though he knew from Ultra intercepts that it was about to be heavily bombed. Declassified material has revealed that he never had such information in the first place, but the phrase remains; nobody knows what Churchill would have done if he had really faced a Coventry dilemma.