But it is necessary to know well how to disguise this characteristic, and to be a great pretender and dissembler; and men are so simple, and so subject to present necessities, that he who seeks to deceive will always find someone who will allow himself to be deceived.Therefore it is unnecessary for a prince to have all the good qualities I have enumerated, but it is very necessary to appear to have them. And I shall dare to say this also, that to have them and always to observe them is injurious, and that to appear to have them is useful; to appear merciful, faithful, humane, religious, upright, and to be so, but with a mind so framed that should you require not to be so, you may be able and know how to change to the opposite.
And you have to understand this, that a prince, especially a new one, cannot observe all those things for which men are esteemed, being often forced, in order to maintain the state, to act contrary to faith, friendship, humanity, and religion. Therefore it is necessary for him to have a mind ready to turn itself accordingly as the winds and variations of fortune force it, yet, as I have said above, not to diverge from the good if he can avoid doing so, but, if compelled, then to know how to set about it. For this reason a prince ought to take care that he never lets anything slip from his lips that is not replete with the above-named five qualities, that he may appear to him who sees and hears him altogether merciful, faithful, humane, upright, and religious. There is nothing more necessary to appear to have than this last quality, inasmuch as men judge generally more by the eye than by the hand, because it belongs to everybody to see you, to few to come in touch with you. Every one sees what you appear to be, few really know what you are, and those few dare not oppose themselves to the opinion of the many, who have the majesty of the state to defend them; and in the actions of all men, and especially of princes, which it is not prudent to challenge, one judges by the result."
Machiavelli, The Prince CHAPTER XVIII "Concerning The Way In Which Princes Should Keep Faith"
Hitler knew Machiavelli's teachings well.