Timothy Snyder's Bloodlands: Europe Between Hitler and Stalin is indispensable. The area was a tough neighborhood even before the two dictators arrived. Borderlands are that way. But modern industrialized killing made everything much worse. George Kaufman makes a similar point. He writes about Flashpoints and updates the story in order to address today's many conflicts in Europe. It's always a "new ballgame" and (Friedman notes) it's not.
Both authors make the point that Hitler and his band murdered millions in the name of race war; Stalin and his followers murdered millions in the name of class war. But Stalin and his heirs (Mao, Ho, Fidel, Che, Hugo, to name a few) still get a better press than the Hitlerites because race war has (rightly) become unfashionable (exclude the jihadists) while class war is still very popular. (What would politicians do without it?)
Writing in the current Independent Review, Andrei Znamenski ("From 'National Socialists' to 'Nazi': History, Politics and the English Language"), notes that Nazis did not refer to themselves as "Nazis." They liked to be called National Socialists. But the label "Nazi" is now preferred by most people in the West so as not to taint the label "socialist". There is supposedly honor in class war so we must be careful who to include in any "socialist" big tent.