Napoleon, Hitler, and Babylon Five. They all relate. I'll try to explain how without giving away too very much....
In the first two books, Harry Potter had been the incidental person breaking up the plans of Lord Voldemort, the most powerful evil wizard on the planet. Each book getting progressively darker (and Rowling says it continues to get darker and more complicated as it moves on) as well. In the third book, he is basically introduced to the last of the truly major players in the overall story, and learns much of the history he needed to know - along with the prophecy of the return of Voldemort to even greater power than before. Now, in the fourth book, Harry is the very specific target of an intricate plot to achieve his assassination and Voldemort's restoration all with the same blow!
Somehow, Rowling finally makes it easy to understand Harry so much better now, with the way things run in this novel. Harry is a war orphan, surviving in the aftermath of what in the wizarding community amounted to a World War. With Voldemort acting the part of Hitler, and plotting a very Napoleonesque return to power, and Harry being the very key to both his success and his failure. The political forces of the world's wizarding community have been in some turmoil ever since Voldemort's initial defeat, and early in the book there is actually a battle between his supporters and the rest, with an altogether unexpected result - one which effectively sets the tone for the rest of the book.
I won't spoil anything more for you, except to say how I could mention Babylon Five in this. The story is the fulcrum for the series; how could it be so? Well, for those who don't know B5 I'm sorry - I really cannot think of any better analogy than this, and anything other than the analogy would take me volumes to say. For those who do though, just two words explain enough, and hopefully leave much to draw you in regardless:
'Nuff said. Hope you enjoy the book as much as I did!