WE know, and never need to see, what became of Muadh al-Kasabeh.

Because no guesswork is required, we know the reaction of the Jordanian pilot's family, friends and compatriots. We also know, if it was ever in doubt, that the insurgency calling itself Islamic State has adopted crimes against humanity as instruments of policy.

To burn a prisoner alive is to break all bounds, to defy anyone's version of the laws, so called, of war. It should hardly need to be added that the act was a greater offence to Islam than anything contrived by the enemies of the faith. What's worse: a murder committed by those lost to reason, or a deed calculated to horrify and - the familiar rationale - terrorise?

We know where we stand. IS is pleased with its work, the effect, and all those hideous little internet celebrations of a death cult. Even the claim to be working towards a caliphate - whatever that means - is a fraud. These people are in the apocalypse business: first destroy all, then construct your theocratic fantasy. If thousands are destroyed in the process, the fact is accounted trivial. IS should be judged by that.

Against the cult we have arguments, armaments and obligations. The last of these isn't slight. In 1915, the Allied Powers in the Great War first talked of crimes against humanity in the context of what the Ottomans had done to 1.5 million Armenians. The phrase reappeared when the time came to formulate charges against Nazis at Nuremberg. The UN recognised apartheid as a crime against humanity almost 40 years ago. It is, in short, the most serious accusation any court can bring.

But there's a twist. It leaves governments uncomfortable and is ignored as often as it is observed. Nevertheless, when "compelling law" is at stake - and crimes against humanity are high on any list - there is no choice in the matter. You don't get to pick and choose, turn away, stand aside, or plead a local political difficulty. When crimes against humanity are being committed, lawful governments everywhere have a duty to act. So the theory goes.

Ironically, the fact is as handy for Western warmongers as it is for IS. Somewhere in his strange mind, Tony Blair is still consoling himself with superhero fantasies of human betterment. Within the pits of their psyches, the killers of a young pilot still maintain that every barbarism has a purpose justified by God. Several hundreds of thousands died because Blair had a direct line to a deity; IS is equally sure of its theology. Yet they ask: people dare to talk about crime?

Someone has to. Then we have to decide what dealing with crimes against humanity might actually involve. Killing innocent villagers with drones and bombs has not counted as our best attempt at an answer. Putting Western boots on the ground, flags flying above, seems to be what IS desires most. The American-led incursions into Iraq and Afghanistan seem, in fact, to have achieved nothing more than a big terrorism problem for the West and an unending list of local jihadi wars. "We" have done no more than provide inspiration for fanatics.

Strategically, that counts as stupid. Equally, getting off the perpetual war treadmill does not solve our ethical problem. Refusing to allow a few tired politicians and their disreputable former advisers to refight old, lost battles in Iraq is not the whole answer to a moral problem. Crimes against humanity have to be answered or the game is up, sooner or later, for all of us. The hellish murder of Muadh al-Kasabeh was a straightforward invitation to a contest.

You would first need to realise that the killers knew as much. Then you would need to grasp that a sophisticated insurgency knows all about Western military power and modern media. You would need to appreciate an IS bet: get those American and British boots back on dusty ground and you might just rouse hosts of young fighters more interested in what they are working against than what they are working for. Such thoughts should pre-occupy "the West".

Instead, we are creeping again towards familiar battlefields. That the development is likely to delight IS seems not to bother many generals. Some even think it a stroke of luck that Britain's major deployment, currently, is against Ebola in Sierra Leone. All of a sudden, after taking our leave of Afghanistan, we have spare killing capacity, an authentically disgusting nominated enemy, and a public that has mostly forgotten about Iraq.

So pause. Remember the previous catastrophes. Think about the purpose of IS goading. Forget the delusion that the world can be reordered at will if you only take extra care, this time, with heavy weapons. Try, just for once, to understand why the Middle East is quite so complicated. Above all, think a bit about why the sight of Western troops produces such an effect across an entire region.

None of that should dissuade anyone from pursuing crimes against humanity. It is striking, though, that those in the West who make speeches are so dishonest with those they represent. The Commons defence select committee, for example, is "surprised and deeply concerned" that the UK has not done more to help crush IS. It seems we should be doing more bombing, deploying more special forces, and sending more troops to train Iraqis and Kurds.

Anyone who doesn't know where that leads has been ignoring a lot of history. For now, in an election year, no politician will dare to commit troops. The generals, patently, are meanwhile deep in the market for a mission likely to creep. The difference between those who train and those who fight can disappear overnight, however, especially when an enemy such as IS wills the outcome.

The Tory Rory Stewart, chair of the committee, has been busy mentioning the UK's responsibilities as a "global power". The language, far less the sentiment, is foolish. Any attempt to aid the rule of international law is eradicated when one of the grand old colonialists decides, as though by habit, to impose its will. The people who live on some parts of the map don't like it.

And why, generals' pensions aside, is it inevitable that our young people must be called up for killing grounds? What is the imperative within our military-industrial complex that requires British blood in places where - for let's not pretend - Britain's troops are not popular? None of the answers are good.

In 2013, the UK was the fifth-largest exporter of arms on the planet. If Jordan's people want revenge for the torture and killing of Muadh al-Kasabeh, therefore, we could probably help. If the Kurdish Peshmerga mean to have a war with IS, we are not short of equipment. If Iraq is ever to be restored as a state, if crimes against humanity are to be halted, the knights, peers and rentiers who infest Britain's arms trade no doubt stand ready.

I wouldn't pass that off as the perfect ethical solution. It would be better, though, than lurching into another Iraq intervention. It would also be the least of desires for the thugs of IS.