He's speaking of a certain (past) moment in time. In that moment in time, Hitler was (apparently) already dead, so verbs referring to his deeds must be in the past perfect tense.
Let me change that to simple past:
This was fertile ground for a new breed of rabble rousing right wing
politicians. Among them Adolf Hitler. Hitler was born in Austria. He
fought bravely as a soldier...
This doesn't sound all too well because he used the simple past to refer to "the time when the ground was fertile for a new breed of..." etc. So he must use a tense that goes further back into the past to explain what happened before those events.
Hope I explained myself.