Lurk on the Internet long enough and you're bound to come across evidence of Godwin's Law, the tongue-in-cheek dictum that "as an online discussion grows longer, the probability of a comparison involving Hitler approaches 1."

But with the rise in the West of the so-called "alt-right" – that chaotic confederation of nativists and white supremacists – does Godwin's Law suddenly have validity? After all, are such comparisons really reductive or inherently fallacious when Americans are flying swastika flags, offering stiff-armed Nazi salutes and paraphrasing Mein Kampf?

In order to understand the use and abuse of such comparisons, The Globe and Mail spoke with German writer and historian Volker Ullrich, the author of the massive new biography of Adolf Hitler (the first volume, Hitler: Ascent, 1889-1939, was published in English translation last year). With its nuanced look at the rise of the Nazi Party (and the resistance to it) and Ullrich's controversial decision to examine Hitler as a human being rather than an avatar of monstrous evil, Ascent proves invaluable in understanding the emergence of the Third Reich and offers some lessons for the present.

With neo-Nazism seemingly on the rise in the West, and America in particular, many people are invoking Nazi Germany as a warning. When your book was reviewed in The New York Times, Michiko Kakutani pulled sections that seemed to directly compare Hitler to Donald Trump. Do you see such comparisons as especially useful?

I do not think comparisons between Hitler and Trump are very useful. There are, of course, some features which both have in common: the egomania, the inclination to mix lies and truth, the promise to restore their own nation to the old greatness, resentment against the elites and so on. But, as a rule, such comparisons are misleading because they lead to the dangerous belittlement of Hitler and the crimes committed under his rule.

The historian Antony Beevor has said that "history is not a predictive mechanism." Do comparisons between Nazi Germany and Trump's America risk fatefully misunderstanding history?

Antony Beevor is right: Drawing lessons from the past for the present is a difficult task, because history never repeats itself. An unreflected comparison between Hitler's Germany and Trump's America actually carries the risk of misunderstanding the great differences. I am referring to, for example, the fact that the American Constitution is based on a system of checks and balances, which a man like Trump will hardly be able to eliminate. In addition, its authoritarian powers are hampered by the strength of American civil society and the influence of the great liberal newspapers. In Germany, after the elimination of the Weimar constitution, the equalization of the parties, associations and media, and the establishment of the dictatorship in 1933-1934, there were no longer any opposing forces which could have stifled Hitler's absolute will to power. ...