IN 1920, the Secretary of State for War, Winston Churchill, suggested that chemical weapons should be used "against recalcitrant Arabs as an experiment". He continued: "I am strongly in favour of using poisoned gas against uncivilised tribes to spread a lively terror" in Iraq. But now the "lively terror" is out of British hands, and the West declares a line to have been crossed. But which line?

As I write, Syria prepares for an air strike from the United States. It is promised some time in the future in response to the use of chemical weapons against anti-government rebels. It is the - possibly reluctant - logic of his own words, spoken in December last year, when President Obama said: "The use of chemical weapons is, and would be, totally unacceptable."

It was a line drawn in the sand, but one supported by the British Foreign Secretary, William Hague, who, writing in The Daily Telegraph, said: "We cannot allow the use of chemical weapons in the 21st century to go unchallenged." So the message is clear: use chemical weapons, and we'll come for you.

On a practical level, a military strike on Syria feels unwise. There are many who believe that it is 1980s Afghanistan all over again, with more than 100 heavily armed groups presently engaged in civil war in the country. Countless jihadis have crossed into the country from Iraq, Kuwait, Algeria, Tunisia, and Pakistan. Witnesses say that "raping and slaughtering pro-Assad regime civilians is becoming the law of the land," and the Free Syrian Army is responsible for many executions. So whose side is the West on, in a many-sided country? And is the ethical argument so clear? Do chemical weapons really present such a clear moral line in the sand?

Chemical weapons are repulsive; but are they more repulsive than conventional warfare? More repulsive than what went on in the Battle of Stalingrad, for instance, in 1943, when 150,000 soldiers died in appalling circumstances? (Of the 91,000 prisoners taken alive, 45,000 died on the march to Siberia, and only 7000 survived to the end of the war.) More repulsive than the genocide in Rwanda in 1994, when between 500,000 and one million Tutsis were murdered by the Hutus? And not a chemical weapon in sight in either of these nightmare settings. Sadly, these are just two examples in a very long list.

The US Secretary of State, John Kerry, calls chemical attack "a moral obscenity". Indeed, but it is a random line in the sand, like saying of a marriage: "Emotional abuse we'll tolerate, but any physical violence and we'll come for you." Churchill believed in the efficacy of chemical attack; but there are many ways to hurt people. Let our national reflections on just intervention not develop a chemical dependence.

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