Opposition groups in Syria maintain that the Assad regime launched a chemical attack on rebel-held areas. The reason this claim cannot be independently verified is that President Assad, through his brutal repression of a country and its captive populations, will not allow disinterested external observers to investigate it.
That is suggestive in itself of the regime's guilt.
What is acknowledged, let alone what is alleged, of Assad's record is already enough to qualify him as one of the worst dictators of modern times.
Yet, the horror of an attack on civilians with chemical agents is a step beyond even that depravity. It breaks a taboo and should be treated as the act of a pariah state, not a potential negotiating partner, by the international community.
It remains an allegation, not a demonstrated fact, that the harrowing video footage released showed victims of a chemical attack by Assad's forces. That is, however, the most likely explanation. Chemical weapons experts identify several characteristics - such as dilated pupils and the struggle of those barely surviving to breathe - that are consistent with such an attack and would be difficult to fake.
While mass murder, for which Assad is unquestionably responsible, is a monstrous crime by any means, inflicting it with manufactured chemical agents is a qualitatively different form of barbarism with an appalling history.
The large-scale use of poison gas in warfare was initiated by Germany at the Battle of Ypres in 1915. There was also an indirect victim: Clara Immerwahr, a brilliant chemist. She was married to Fritz Haber, a German Jew who had developed gas for use in trench warfare and was later awarded a Nobel Prize. Immerwahr instinctively understood the horror of chemical warfare and denounced it. After a row with her husband, she shot herself.
Immerwahr's self-sacrifice was born of conscience and an acute understanding of the nature and consequences of such warfare. The use of gas Ypres killed 5,000 allied soldiers, who were totally unprepared for it. It was followed by the allies' use of poison gas at Loos a few months later. The total casualties of gas, comprising some 25 different agents, in these and other engagements in the First World War numbered hundreds of thousands without even any military utility.
Trench warfare was not ended. The lives of many soldiers and civilians, however, were - lingering and in agony.
In modern times, the terror of chemical attack was employed by a tyrant without scruple or equal. In March 1988, Saddam Hussein's forces attacked the Kurdish town of Halabja in Northern Iraq with poison gas. Some 5,000 Kurds died instantly; thousands more perished in agony later.
Survivors of Halabja recall that some people who inhaled the chemicals stumbled around the town literally blindly, buckling at the knees owing to the attack on their nerve systems.
It was an horrific way to die - and this was but one of at least 40 chemical assaults ordered by the regime in the Iran-Iraq War.
Chemical assault by the Baathist regime marked a form of warfare that was worse than a crime.
It was a form of genocide. In that ignoble company, Assad is not out of place. Diplomacy has proved quite unable to halt his plan to create a wasteland and call it peace.