The truth shall set you free

Listening to the radio whilst running late for work, and there’s an American chap on plugging his book on lying in public life. Moderately interesting chat with him and Kim Howells (sp?).

However, it ended with the gem that one of the biggest lies told in recent times was during the Cold War. Namely Western governments saying that they would use nukes if necessary should a war in Europe be going badly. We know this is a lie, told for deterrence, because when the Cold War ended, relevant Defence Secretaries said they’d never have actually done it.

Which is all very nice. Except, on what basis does one distinguish which statement is the convenient untruth appropriate to the spirit of the times?