THE horrors of Syria’s civil war have become so routine and relentless that recent massacres have barely been aired in the world press. But if it can be verified that on August 21st nearly 1,200 people, most of them civilians, were killed by chemical weapons fired by President Bashar Assad’s forces, a new level of atrocity will have been reached that may persuade Western governments to change their policy of military non-intervention. Barack Obama declared in a speech a year ago that he would “change his calculus” if Mr Assad were to plumb such depths of brutality. Since then, the American president has set his face against direct involvement. Now, however, he and the two Western governments that have come closest to tackling Mr Assad head on, France and Britain, may be badgered into thinking again. The two Arab countries most keenly arming the rebels, Saudi Arabia and Qatar, may also try again to pull the Western trio in. In the early hours of the morning of August 21st, residents reported shelling and rocket attacks on Eastern Ghouta, a swathe of disaffected suburbs to the east of the capital, Damascus. Shortly afterwards, gruesome videos were posted showing victims struggling to breathe and displaying other symptoms of chemical poisoning—possibly by a nerve agent, sarin.

Could the rebels have faked such videos, as the regime’s spokesmen promptly accused them of doing? Independent experts’ early assessments, albeit tentative, tended to judge the reports credible. The poison shells are said to have been fired in areas where rebels have maintained a stubborn resistance to the regime, whose forces conducted air raids and bombarded them on the same day. Syria is thought to have hundreds of tonnes of mustard gas, sarin and the far more lethal VX, as well as other poisons not banned under the Chemical Weapons Convention signed by nearly all the world’s countries.

The attack, if verified, is peculiarly ill-timed, since a UN team of weapons inspectors arrived in Damascus only three days earlier to investigate previous allegations of chemical weapons being used. The regime is most unlikely to let the UN people visit the sites of the latest suspected attack. If it were proved to be chemical, it would be the deadliest of its kind since Iraq’s Saddam Hussein dropped poison bombs on Kurds in the town of Halabja in 1988, killing 5,000 of them.

It is not the first time the Syrian regime has been accused of using chemical weapons. The UN inspectors are due to visit three places, including Khan Assal, a town west of Aleppo, where 30-odd people died in mysterious circumstances in March. The inspectors have been asked not to find out who carried out the attacks but to verify whether chemical weapons were used. The governments of Britain, France and Israel have told Ban Ki-Moon, the UN’s secretary-general, that their intelligence services are pretty sure Mr Assad has used chemical weapons on a small scale to test the West’s willingness to respond militarily, with (for him) pleasingly negative results. He has also tested the West’s resolve by firing SCUD missiles into civilian-populated areas, a war crime, which likewise has failed to provoke a more muscular response from the West. The latest escalation, if true, suggests Mr Assad’s growing conviction that he can thumb his nose at the West with impunity.

After the rebels lost the town of Qusayr two months ago, Mr Assad’s forces seemed to be on a military roll. More recently, however, the rebels have fought back, taking an airbase near Aleppo, consolidating in large parts of the north and east, and even attacking an area just north of Latakia, near Mr Assad’s own clan heartland.

But as Islamist extremists become more dominant in rebel ranks, Mr Assad has increasingly been seen as the lesser of evils, both by religious minorities in Syria such as Christians, and hostile Western and Arab governments. Mr Obama’s reluctance to intervene in Syria, like his unwillingness to call the overthrow of the Muslim Brotherhood’s Muhammad Morsi in Egypt a coup, may have further emboldened Mr Assad. Russia, his stalwart ally, was quick to back his regime’s denial that it had used chemical weapons, hinting that Britain’s push to refer the matter to the UN Security Council would be blocked

“The beauty of chemical weapons”, in Mr Assad’s view, says Emile Hokayem of the London-based International Institute for Strategic Studies, “is that there is enough ambiguity to allow discussion of a response to drag on for weeks or months.” By that time, Mr Assad hopes, he may have the rebels back on the ropes. Or he may have made a drastic miscalculation.