Syria’s renewed use of chemical weapons against its own people at the weekend is shameless and barbaric. Dozens of people in the remaining rebel-held suburbs of Damascus were suffocated by Saturday’s chemical attack on the Douma district. This is not the first time this has happened. Since the use of sarin at Khan al-Assal in 2013 there have been dozens of chemical attacks by the regime. These deliberate attacks on civilians show callous contempt for humanity and disregard for the laws of war. Official Syrian claims that the latest killings have been fabricated are beneath contempt.

Yet Bashar al-Assad has again used chemical weapons for two reasons that shame others, as well as him. First, he has done it because he has the means and the will. Second, he has done it because he knows he can get away with it. His crimes are his own. But they have been made possible, among other things, by the failure of any effective legal, diplomatic and military sanctions.

Some may ask why, since the slow throttling of Damascus’s eastern Ghouta suburbs seems to be approaching a grisly climax, the government feels any need to breach one of the oldest taboos in warfare once more. To answer that adequately it is necessary to delve into the darkest places of the psychology of a regime that celebrates the overwhelming use of force, the need to terrorise civilians and the right to punish opponents indiscriminately as a weapon of policy.

It should come as no surprise, though, not least in the light of the Skripal poisoning, that Russia bears a major share of responsibility. The Syrian air force was able to bomb Douma because Russia controls western Syria’s airspace. Russian advisers are present at the airbases from which Syrian missions fly. The Russians may not be closely involved in individual Syrian decisions. But they provide active military and diplomatic cover for the Assad regime’s use of chemical weapons. Russia’s implacable veto at the United Nations over any effective countermeasures has provided a green light to the Assad regime to kill its own children.

Yet American policy is scarcely more defensible, either in general or in the specific case of Saturday’s attack. In the wake of Iraq, US policy was indecisive under Barack Obama. It is now downright chaotic under Donald Trump. A year ago, after a chemical attack by the Syrian government killed dozens of people in Khan Sheikhun, Mr Trump sprayed 59 cruise missiles on the airbase from which the attack missions had been flown. Since then, US policy on Syria has repeatedly flip-flopped, especially towards the Kurds. In January, the then secretary of state, Rex Tillerson, pledged that US forces would remain in Syria well after any defeat of so-called Islamic State. A week ago Mr Trump countermanded that, saying “it’s time” to bring US troops home. A day later he changed again, saying the troops would stay for months. It would hardly have been surprising if the Assad regime sensed an opportunity. On Sunday Mr Trump threatened that Mr Assad would pay a “big price”. But the truth is that the US is increasingly marginalised.

The imminent fall of eastern Ghouta will not mark the end of the Syrian conflict. It is more likely to mark a new phase. With Damascus more secure, and Isis quiescent at least temporarily, the country will be effectively divided in three. The Assad regime is sponsored by Iran and Russia; the Kurds are still under the uncertain protection of the US; an area north of Aleppo is Turkish-controlled. This is hardly more stable than when the anti-Assad revolt began in 2011. Without an unlikely diplomatic solution, the tragic deaths in Douma are unlikely to be the last.