Michael Vassar hit upon something which I think is probably more key than rationality: Charisma.
Hitler had goals, and by the means he had at his disposal, he was highly rational in achieving those goals (this does not mean that he was rational in the sense of being sane). I also recall from hearing my Great-Great Uncle talk about meeting Hitler once, that he said the man could talk a person into anything (he also related how devastated he was when he learned of how evil the man was. Of course, just not liking Jews was not considered to be exactly evil in much of Europe at the time either). Please, do not read this as apologizing for Hitler's actions. It is just a statement that he was a very skilled personal operator, and as someone later has stated, he managed to get other people to do his work for him.
That seems to be the MO of almost all politicians: To be able to personally charm those they meet, and then subtly suggest "You can help me if you go out and tell others about my GRAND PLAN" (Wasn't there a post about The One Big Theory? This seems to be among the tools of the politically motivated. That they have One Big Theory that will "fix it all").
That "One Big Theory" is so appealing to most people that they are willing to overlook the failures of the theory in order to feel god. It is exactly like a drug.
Hitler had goals, and by the means he had at his disposal, he was highly rational in achieving those goals (this does not mean that he was rational in the sense of being sane).
What is the meaning of "sane", then? Conforming to the majority?
As Eliezer and many others on Less Wrong have said, the way the human species rose to dominate the Earth was through our intelligence- and not through our muscle power, biochemical weapons, or superior resistance to environmental hazards. Given our overwhelming power over other species, and the fact that many former top predators are now extinct or endangered, we should readily accept that general intelligence is a game-changing power on the species level.
Similarly, one of the key ingredients in the birth of the modern era was the discovery of science, and its counterpart, the discovery of the art of Traditional Rationality. Armed with these, the nations of Western Europe managed to dominate the entire rest of the world, even though, when they began their explorations in the 15th century, the Chinese were more advanced in many respects. Given how Western Europe, and the cultures derived from it, has so completely surpassed the rest of the world in terms of wealth and military might, we should readily accept that science and rationality is a game-changing power on the civilization level.
However, neither of these imply that intelligence, science, and rationality, as a practical matter, are the best way to get things done by individual people operating in the year 2009. We can easily see that many things which work on the species level, or the civilization level, do not work for individuals and small groups. For instance, until the discovery of nuclear weapons, armed conflict was often a primary means of settling disputes between nation-states. However, if you tried to settle your dispute with your neighbor, or your company's dispute with its competitor, using armed force, it would achieve nothing except getting you thrown in prison.
People are crazy and the world is mad, but it does not necessarily follow that we should try to solve our own problems primarily by becoming more sane. Plenty of people achieve many of their goals despite being completely nuts. Adolf Hitler, for example, achieved a large fraction of his (extremely ambitious!) goals, despite having numerous beliefs that most of us would recognize as making no sense whatsoever.
We know, as a matter of historical fact, that Adolf Hitler and his Nazi Party, despite being generally incompetent, unintelligent, irrational, superstitious and just plain insane, managed to take over a country of tens of millions of people from nothing, in the span of fifteen years. So far as I am aware, no group of people has managed to achieve anything even remotely similar using, not only rationality, but any skill involving deliberative thought, as opposed to skills such as yelling at huge crowds of people. However, it is a corollary to the statement that no one knows what science doesn't know that no one knows what history doesn't know, so it is entirely possible, perhaps likely, that there is something I am overlooking. To anyone who would assert that intelligence, science or rationality is the Ultimate Power, not just on the level of a species or civilization, but on the level of an individual or small group, let them show that their belief is based in reality.