Context.
Hindsight is 20/20....and do keep in mind that at that time Hitler was far from being the only anti-semitic leader in Europe.
They were far more concerned about Bolshevism (and justifiably so), and they saw at the time Hitler as a buffer against it.
Mein Kampf was published in 1923. It laid out the entire vision of what Hitler wanted to do. It is not hindsight to say that Germany should have known. the truth is that Europeans have hated Jews for a very very long time (and frankly still do). They would do anything to rid themselves of the problem which is why Hiitler was so attractive.
To think that there is a fundamental difference between NAZI thought and the ‘bolshveks’ is nonsense. It demonstrates a lack of understanding of both forms of government. They are BOTH totalitarian in nature. They are anti-Christian as well as anti-Semitic. The rhetoric may sound different but both are anti-freedom and anti-liberty.