Question:
Where do you stop telling the truth and start diplomacy?
Answer:
Some knowledge and truth is dangerous to the many, and thus can only be safely given to the few on a need to know basis. So, I would have to say that diplomacy starts where you need to fill in the gaps left behind after the whole truth has been diplomatically altered or pruned of potentially dangerous content.
To believe that all truth wants to be free and should thus be made freely available to everyone is a laudable but naive ideal, especially when it comes to metaphysics.
Diplomacy can be likened to what you might tell your children while at the beach. Your children might ask you if there are any sharks at this beach.* You know shark encounters are remotely possible because you are facing the open ocean, which is well known to contain sharks. But you consider the risks to be minimal and acceptable.* So, instead of telling the whole truth and terrifying your children, you reassure them by telling them that there are no sharks at that beach.