I just finished reading "The Ominous Parallels" by Leonard Peikoff and I can't recommend it highly enough. It is subtitled "The end of freedom in America." The parallels of course are those ideas that exist in America today and the German people in the 40 or so years leading up to the arrival of the Nazis and Hitler.
He showed how pragmatism destroyed the Germans' ability to think in terms of principles just as it's doing in America today, and how it reduces a human mind to the perceptual level of animals. But it isn't just pragmatism. Dr Peikoff discusses dogmatism, subjectivism--both individual and social versions, nihilism, romanticism, relativism and others.
I've heard people say that Hitler was insane, a madman. This is a mistake. Hitler was loyal to his principles. He and his lieutenants studied philosophy, mostly that of Plato, Kant and Hegal on whose principles Nazism was based.
If there was ever any doubt about the power of philosophy to create of destroy an entire nation, this book will remove it.