By the skillful and sustained use of propaganda, one can make a people see even heaven as hell or an extremely wretched life as paradise. -- Adolph Hitler
This quote epitomizes Hitler's philosophy as well as his plan to exterminate entire groups of people (primarily the Jews) and take over the world. If anyone had really been listening, they might have determined his intention from this quote.
Hitler succeeded in his quest (as far as it went) largely because of propaganda. He was an effective speaker and began, long before he had any real power, to characterize the Jews as the root of all evil, even blaming them for profiteering during World War I and causing Germany to lose the war.
Hitler is also famous for saying that “if you tell a big enough lie and tell it frequently enough, it will be believed," and that is exactly what happened. Hitler used his speeches, his book, and his increasingly significant role in government to prepare Germany for his Final Solution. He begins with such innocuous statements as "removing the Jew" and eventually ends with a holocaust.
Hitler encounters very little resistance, given the outrageousness and scope of his actions. His patience pays off and the groundwork of propaganda he set allows him to become a dictator in a totalitarian regime which embodies arguably the worst atrocities of the twentieth century.
As the above quote indicates, Hitler did warn people of his intentions, but no one--or at least not enough "someones"--believed him.