Their propaganda is marked by its extreme contempt for facts as such, for in their opinion fact depends entirely on the power of man who can fabricate it.

From The Origins of Totalitarianism Hannah Arendt. Chapter 2.  Emphasis added.

The propaganda effect of infallibility, the striking success of posing as a mere interpreting agent of predictable forces, has encouraged in totalitarian dictators the habit of announcing their political intentions in the form of prophecy. The most famous example is Hitler's announcement to the German Reichstag in January, 1939: "I want today once again to make a prophecy: In case the Jewish financiers ... succeed once more in hurling the peoples into a world war, the result will be ... the annihilation of the Jewish race in Europe."23 Translated into nontotalitarian language, this meant: I intend to make war and I intend to kill the Jews of Europe. Similarly Stalin, in the great speech before the Central Committee of the Communist Party in 1930 in which he prepared the physical liquidation of intraparty right and left deviationists, described them as representatives of "dying classes." This definition not only gave the argument its specific sharpness but also announced, in totalitarian style, the physical destruction of those whose "dying out" had just been prophesied. In both instances the same objective is accomplished: the liquidation is fitted into a historical process in which man only does or suffers what, according to immutable laws, is bound to happen anyway. As soon as the execution of the victims has been carried out, the "prophecy" becomes a retrospective alibi: nothing happened but what had already been predicted.  It does not matter whether the "laws of history" spell the "doom" of the classes and their representatives, or whether the "laws of nature ... exterminate" all those elements—democracies, Jews, Eastern subhumans (Untermenschen), or the incurably sick—that are not "fit to live" anyway. Incidentally, Hitler too spoke of "dying classes" that ought to be "eliminated without much ado."

This method, like other totalitarian propaganda methods, is foolproof only after the movements have seized power. Then all debate about the truth or falsity of a totalitarian dictator's prediction is as weird as arguing with a potential murderer about whether his future victim is dead or alive—since by killing the person in question the murderer can promptly provide proof of the correctness of his statement. The only valid argument under such conditions is promptly to rescue the person whose death is predicted. Before mass leaders seize the power to fit reality to their lies, their propaganda is marked by its extreme contempt for facts as such, for in their opinion fact depends entirely on the power of man who can fabricate it. The assertion that the Moscow subway is the only one in the world is a lie only so long as the Bolsheviks have not the power to destroy all the others. In other words, the method of infallible prediction, more than any other totalitarian propaganda device, betrays its ultimate goal of world conquest, since only in a world completely under his control could the totalitarian ruler possibly realize all his lies and make true all his prophecies.