Adolf Hitler and Joseph Stalin, the two most powerful personifications of evil in this century, are still impossible to explain fully. They shouldered their way into politics as resentful, hate-filled egoists, but so did thousands of their contemporaries. To anyone scrutinizing the young Hitler or Stalin, writes Alan Bullock, the Oxford University historian, "a suggestion that he would play a major role in twentieth-century history would have appeared incredible." At 30, Hitler was a street-corner speechmaker in Munich, and Stalin was in prison for plotting an oil workers' strike in Baku.
"They developed over time," says Lord Bullock -- he became...