Because it was the first truly global war, and because its effects continue to be felt in world politics, the causes, conduct, conclusion, and consequences of World War II have been deeply, and sometimes bitterly, disputed for fifty years. One classic interpretation of the war was fixed early on, with the publication of The Gathering Storm, the first in Winston Churchill's six-volume history The Second World War. According to Churchill, World War II was "The Unnecessary War": stupid Allied peacemaking after World War I, followed by pusillanimous Allied policy, led to the second phase of a new Thirty Years' War. But while Churchill held the negotiators at Versailles and the British and French leadership of the 1930s responsible for egregious errors of commission and omission, he was also certain that the Second World War would not have happened absent "a maniac of ferocious genius, the repository and expression of the most virulent hatreds that have ever corroded the human breast—Corporal Hitler."

Some years later, A. J. P. Taylor, a British historian of a different ideological disposition, caused a furor by adopting half of Churchill's analysis: the ineptitudes of the World War I victors remained, but the "maniac of ferocious genius" was transformed into a rational (or, at the very least, purposeful) politician who, having made his aims plain for all to see, could not be blamed for taking advantage of the vacillations and strategic failures of the Baldwin government and the revolving cabinets of the French Third Republic. It was an argument well received in the iconoclastic sixties.

More recently, less pretentiously, and more persuasively, the German-émigré historian Gerhard Weinberg, now of the University of North Carolina, has challenged the "New Thirty Years' War" image, arguing that, despite some obvious similarities, the Second World War was a very different business from the First: