Two of the grimmest places I have ever visited are the Tuol Sleng Genocide Museum and the Choeung Ek killing fields in Cambodia, dedicated to the memory of the hundreds of thousands of victims of the Khmer Rouge. They have been much in my mind this week.
So, too, has the Paneriai forest outside Vilnius, in Lithuania, which I visited last summer, where 100,000 Jews were murdered between 1941 and 1944. I also visited Kaunas, where my grandmother was one of tens of thousands of people shot by SS death squads in 1941. Massacres, genocide, horrors beyond imagination. Which brings us to the equally unimaginable horror of IS, or ISIS, or ISIL, in Syria and Iraq.
Whatever you call them, their brutality is so shocking that it leaves us struggling to find the words with which to express our revulsion. Unlike the Nazis or the Khmer Rouge - or other mass killers like Stalin or Mao - they are so proud of their cruelty that they film it and broadcast it, for all to see.
I hope to God you didn't watch the video of the immolation of the Jordanian pilot Muadh al-Kasasbeh, which his killers published this week. To watch it, or to republish it, is to do exactly what they wanted you to do. So thank you, Fox News, for plumbing new depths of idiocy by putting the video, in its entirety, on their website. (Still, from the people who brought you the "Muslims only" city of Birmingham, we should expect no better.)
I am not suggesting that IS have slaughtered as many people as those responsible for the massacres of 1940s Europe or 1970s Cambodia. My point is not to draw an arithmetical parallel, but to ask this question: at what point does our knowledge of such cruelty and such suffering compel us, in the name of morality, to take action to stop it?
Debate still rages about whether 70 years ago the Allies should have bombed the railway lines that led to the Nazi death camps. In Rwanda, we did nothing as an estimated 800,000 people were slaughtered in just a few weeks; but in Bosnia and Kosovo we did send in troops to put an end to mass killing and ethnic cleansing. We can act, sometimes; and we do act. Sometimes.
Is this one of those times? The House of Commons defence select committee certainly thinks so. In a highly critical report published on Thursday, the committee said: "We are surprised and deeply concerned that the UK is not doing more."
(At the time of the MPs' visit to Iraq last December, "the entire UK military presence in Iraq, outside the Kurdish regions, amounted to three individuals. By comparison the Australians have offered up to 400 troops, the Spanish 300 troops, and Italy 280.")
Their report concludes: "We are not calling for combat troops, still less for an attempt to repeat the counter-insurgency and state-building agendas of Iraq in 2007. Any contemporary intervention must be far more focused and incremental. But this is not a reason for the UK to lurch from over-intervention to complete isolation."
And there you have it. The appalling mistakes of the Iraq invasion have now burrowed so deeply into the political pysche that policy-makers tend to recoil instinctively from any suggestion of renewed military intervention in the region. As we saw when MPs refused to authorise action in Syria in August 2013 - not necessarily, by the way, the wrong decision - they are much more than twice shy having been bitten in both Iraq and Afghanistan.
We need to liaise closely with Jordan, which borders not only both Syria and Iraq, but also Saudi Arabia and Israel. How's that for a tricky neighbourhood? With a population of around 6.5 million, it's also now home to well over a million Syrian refugees. (Multiply that by 10 to get some idea of what that would mean in the UK. 10 million refugees? Hmm...)
King Abdullah's late father, King Hussein, was known to British diplomats as the PLK -- the plucky little king. It was both patronising and admiring, but Jordan does have exceptionally close ties to the UK and has, against considerable odds, managed somehow to juggle the demands of all its neighbours. It's one of only two Arab states (the other one is Egypt) to have signed a peace treaty with Israel.
(By the way, here's a little known fact for you: in 1996, three years before he became king, Abdullah appeared as an extra in an episode of Star Trek.)
Now, after the brutal murder of its captured pilot, Jordan says it intends to step up its air strikes against IS targets and to defeat "this terrorist organisation [that] is not only fighting us, but also fighting Islam and its pure values." Its warplanes have already been in action again in both Syria and Iraq.
It will be neither quick nor easy. But for a Sunni Muslim nation to take a leading role in the international fight against IS would be no bad thing. (The Hashemites who rule Jordan claim direct descent from the prophet Mohammad.) The most useful role for the UK and other foreign powers would be to assist, with logistical, training and intelligence support, the efforts of Jordanian, Iraqi and Kurdish forces to defeat IS.
Not only because it is a brutal, murderous sect threatening vital UK interests by infecting British-born fighters and others with the virus of its perverted ideology (according to the Commons defence committee, IS "provides safe haven to an estimated 20,000 foreign fighters ...") but equally because it is bringing misery to tens of thousands of people in both Iraq and Syria who now live in areas controlled by the sect.
It can be done, and it should be done. None of us will be able honestly to say to future generations: "But we didn't know."