If there's one good thing to have come out of the debate over Labour and antisemitism, it might be the emerging consensus that, when discussing the Middle East, it's best to leave Hitler out of it. And not just the Middle East. Boris Johnson was widely mocked for likening the EU to Hitler's ambitions for Europe, leaving open the possibility that Sadiq Khan might become the first ever London mayor not to make crass references to the Führer.
Still, the casual Hitler comparison is especially toxic when discussing Jews and Israel. It's not just that the charge cannot be sustained - no matter how badly you feel Israel is behaving, it is not guilty of the crimes of the Third Reich - it's also designed to hurt Jews in their most sensitive spot. This is why the near-universal condemnation of Ken Livingstone's declaration that Hitler was a supporter of Zionism is heartening: it suggests people understand how cruel it is to put Jews on the same moral plane as their most murderous persecutors, whether by accusing them of committing an equal horror or suggesting, as Livingstone did, that they colluded with their killers as ideological comrades.
I wish I could say I was blameless on this score, but I can't. Sixteen years ago, I was appalled by a short book called The Holocaust Industry by Norman Finkelstein. I wrote that it echoed arguments made by David Irving, who had just lost his notorious libel action and had been branded by the High Court as nothing more than a "pro-Nazi polemicist". Finkelstein's book praised Irving as having made an "indispensable" contribution to our understanding of the last war. In the final line of the piece I wrote that Finkelstein's outlook took "him closer to the people who created the Holocaust than to those who suffered in it."
I now regret writing that sentence. Finkelstein is a child of Holocaust survivors but even if he were not, I should not have written those words. If I could withdraw them, I would. Implicitly, I had made the comparison - of Jews and Nazis - that I believe should be off-limits.
And yet, drawing that boundary is not as easy or absolute as we might like. Not long after Livingstone's outburst, Maj. Gen. Yair Golan, Deputy Chief of Staff of the Israeli military, told a Yom Hashoah ceremony he saw troubling signs in Israel of the "horrific processes" - of "intolerance, violence and… moral degradation" - that had unfolded in Germany in the 1930s. Last Friday, former Prime Minister Ehud Barak declared that Israel had been "infected by the seeds of fascism".
Naturally, some of Livingstone's initial defenders seized on those remarks to suggest their man was in the clear: if these Israelis could say that, why couldn't he say what he liked?
But the sentiments were completely different. Neither Golan nor Barak were twisting the historical record to suggest Nazis and Zionists were partners in a shared enterprise, as Livingstone had done. They were sounding the alarm - the ultimate alarm - about what's happening in their country today.
And that's the key difference: intention. Golan and Barak were not engaged in scoring points. They were not trying to inflict hurt on Jews, by poking into their deepest wound. They were not taking gleeful pleasure in Jewish anguish. On the contrary, they invoked the precedent of the 1930s because they wanted to warn the country they love - and which they have served - off the dangerous path they fear it is taking. It's not a parallel I would want to invoke. But if it's ever to have a legitimate use, then used this way, by these people, is probably it.
Jonathan Freedland is a columnist for The Guardian