Plato's "Grand Lie" - a.k.a. "Noble Lie"

Lie as a tool. Dangerous in the contents, and in the method of imposing it.

"The rulers then of the city may, if anybody, fitly lie on account of enemies or citizens for the benefit of the state; no others may have anything to do with it but for a layman to lie to rulers of that kind we shall affirm to be as great a sin, nay a greater, than it is for a patient not to tell physician or an athlete his trainer the truth about his bodily condition, or for a man to deceive the pilot about the ship and the sailors as to the real condition of himself or a fellow-sailor, and how they fare." (Republic 389b-c)
...

"...How then may we devise one of those needful falsehoods of which we lately spoke—just one Grand lie which may deceive the rulers, if that be possible, and at any rate the rest of the city? What sort of lie? he said. Nothing new, I replied; only an old Phoenician tale of what has often occurred before now in other places, (as the poets say, and have made the world believe,) though not in our time, and I do not know whether such an event could ever happen again, or could now even be made probable, if it did.

...I will speak ... the audacious fiction, which I propose to communicate gradually, first to the rulers, then to the soldiers, and lastly to the people. They are to be told that their youth was a dream, and the education and training which they received from us, an appearance only; in reality during all that time they were being formed and fed in the womb of the earth, where they themselves and their arms and appurtenances were manufactured; when they were completed, the earth, their mother, sent them up; and so, their country being their mother and also their nurse, they are bound to advise for her good, and to defend her against attacks, and her citizens they are to regard as children of the earth and their own brothers.

... Citizens, we shall say to them in our tale, you are brothers, yet God has framed you differently. Some of you have the power of command, and in the composition of these he has mingled gold, wherefore also they have the greatest honour; others he has made of silver, to be auxiliaries; others again who are to be husbandmen and craftsmen he has composed of brass and iron; and the species will generally be preserved in the children. But as all are of the same original stock, a golden parent will sometimes have a silver son, or a silver parent a golden son. And God proclaims as a first principle to the rulers, and above all else, that there is nothing which they should so anxiously guard, or of which they are to be such good guardians, as of the purity of the race. They should observe what elements mingle in their offspring; for if the son of a golden or silver parent has an admixture of brass and iron, then nature orders a transposition of ranks, and the eye of the ruler must not be pitiful towards the child because he has to descend in the scale and become a husbandman or artisan, just as there may be sons of artisans who having an admixture of gold or silver in them are raised to honour, and become guardians or auxiliaries. For an oracle says that when a man of brass or iron guards the State, it will be destroyed.

Such is the tale; is there any possibility of making our citizens believe in it? Not in the present generation, he replied; there is no way of accomplishing this; but their sons may be made to believe in the tale, and their sons' sons, and posterity after them. 
(Republic 414b ...)

Follow up...

The observation of intellectual Theodore Dalrymple offers a concrete instance of the application and consequences of this principle:

In my study of communist societies, I came to the conclusion that the purpose of communist propaganda was not to persuade or convince, nor to inform, but to humiliate; and therefore, the less it corresponded to reality the better. When people are forced to remain silent when they are being told the most obvious lies, or even worse when they are forced to repeat the lies themselves, they lose once and for all their sense of probity. To assent to obvious lies is to co-operate with evil, and in some small way to become evil oneself. One’s standing to resist anything is thus eroded, and even destroyed. A society of emasculated liars is easy to control. I think if you examine political correctness, it has the same effect and is intended to. 

(apparently in) Frontpage magazine (can't find exact reference)