One of the strongest arguments against leftist opponents of liberal intervention has always been a historical one. If you followed their reasoning to its logical conclusion, and applied it retrospectively, then the western democracies would never had stood up to Hitler. And there's nothing more damning for a good leftist than to be ranked among the appeasers (even though there were many in the 1930s, on the left as well as the right, who argued that cutting a deal with Hitler would be less awful than going to war, and who suggested - pre-echoing many on the 'indecent left' today - that Nazism was not much worse than western capitalism).
So it was probably only a matter of time before someone on the 'stopper' left attempted to undermine this historical argument. It seems that the experimental novelist Nicholson Baker, whose expertise as a historian was hitherto unknown, has gone there, with his book Human Smoke: The Beginning of World War II, the End of Civilisation. Baker suggests that the world would have been a better place if we'd just let Hitler get on with things: fewer lives would have been lost, the occupation of Europe wouldn't have been so bad, etc. Most contentiously, the book argues that if the Allies hadn't intervened, then the Holocaust might not have happened, thus neatly adding the worst genocide in history to the roll-call of things for which the west is 'really' to blame.
Anne Applebaum does a good job of demolishing Baker's arguments in her review in The New Republic, in which she accuses him of basing his analysis on second-hand sources and recycled myths taken from the internet. Christopher Hitchens is equally trenchant in his New Statesman review, criticising among other things the tastelessness of Baker's title. However, the general tenor of the comments in response to Hitchens' piece, on the NS website, which tend to favour Baker and repeat all the predictable non-arguments about Hitch's 'neocon' connections, confirm that the book is not an eccentric one-off but one more symptom of a wider malaise of the western left.