BARACK OBAMA came to office vowing to end wars in the Muslim world, not start them. For two years he has resisted calls, which close advisers have made with passion, to intervene in Syria’s ever-more-bloody civil war. And yet, as The Economist went to press, he stood poised to launch an attack on Syria’s armed forces, the results of which could not be foreseen. Even the most cautious American leaders will saddle up and play sheriff if the alternative is a world in which, when America has clearly announced that it will defend an international norm, a rogue dictator thinks he can call its bluff.

Eastern Ghouta, a suburban part of Damascus, was shelled at around 2am on August 21st. Soon people started suffering convulsions and choking, their muscles no longer under their control. Makeshift hospitals began to fill up, with volunteers forced to pour water over the afflicted to avoid contamination.

Médecins Sans Frontières, a charity, said three clinics it supports in the area treated 3,600 patients in a matter of hours, 355 of whom died. The Violations Documentation Centre, a Syrian organisation meticulous in its compilation of reports of death and injury, now puts the death toll at 457 or more. Other credible estimates range as high as 1,300. Harrowing videos—a man begging his two dead children to get up and walk; a girl repeating in wonder “I’m alive, I’m alive”—brought the atrocity home to the world.

The immediate assumption was that Syria’s armed forces were responsible. They have 1,000 tonnes of chemical weapons in their stockpiles, including the nerve agents Sarin and VX. These produce effects like those seen in the attack, though the symptoms seen do not quite match what textbooks would lead you to expect. A clearer picture of the agents involved should emerge from the work of a United Nations (UN) inspection team. In the country to investigate claims of earlier chemical attacks, the inspectors were at first denied access to the affected areas, then fired at by snipers. Having at last visited Eastern Ghouta, they may report on what they have seen in the first days of September.

A guilty look

Chemical weapons do not have to be used by the people who make and stockpile them. But from early on other evidence pointed the finger at the regime led by Bashar Assad. A wide area was attacked; precise rockets were used; the regime continued to fire on the site of the attack for days afterwards. It all seems beyond the capacity of the rebels. Captain Alaa al-Basha, a rebel commander in Damascus, thinks the regime was trying to depopulate the area in the face of renewed progress by rebels pushing towards the centre of the capital. Others point to similarities with earlier, much smaller attacks, and speculate that perhaps this time someone misjudged something.

America and its allies have more than this circumstantial evidence. Intelligence, apparently including telephone intercepts and remote-sensing data, convinced them of the regime’s guilt. America’s secretary of state, John Kerry, decried the “moral obscenity” of slaughtering civilians, including women and children, with chemical weapons. (Indeed, Mr Kerry had favoured military retaliation against the Syrian regime after an earlier, small-scale use of chemical weapons, says a source familiar with the debate inside government.) After long months of meeting atrocities in Syria with hedged rebukes and condemnations-with-caveats, White House spokesmen began throwing around words like “repugnant” and arguing that to allow chemical strikes to be left unanswered would be a threat to America’s national security.

The president’s own credibility is also at stake. On August 20th 2012 he warned Syria that the systematic use of chemical weapons would cross a red line and demand an American response. Although Mr Obama said on August 28th that he had made no final decision, if Mr Assad is to be punished, and both he and others deterred from using chemical weapons, there seems no real alternative to military force.

America has been careful to accuse Mr Assad of breaching “international norms”, rather than the law. Syria is one of just five countries which have not signed the UN’s Chemical Weapons Convention, arguing that Israel’s undeclared nuclear weapons legitimise Mr Assad’s possession of weapons of mass destruction. But there is a case that the convention’s provisions apply anyway as international customary law.

Arab states, such as Saudi Arabia, which are hostile to Mr Assad would favour such strikes, but are wary of heartily backing them in public. On August 27th the Arab League issued a statement which condemned the Syrian regime but stopped short of calling for military action. Ban Ki-moon, the UN chief, wants to give the UN inspectors time to finish their work in Syria, though they will say only whether chemical weapons were used, not who used them; Lakhdar Brahimi, the UN and Arab League envoy to Syria, turned his face against force unless mandated by the UN.

Russia’s foreign minister is reported to have said no UN resolution allowing force should be considered until the inspectors have reported (after which his country would surely veto it). Turkey has said that it is willing to support a strike, even without a UN mandate, if the regime was responsible for the attack. France’s president, François Hollande, said in a speech his country was “ready to punish” the guilty, and Germany, hitherto reticent, has also been supportive. That gives Mr Obama broader European support than George Bush had when he invaded Iraq in 2003.

Britain’s prime minister, David Cameron, who has recently pushed harder for intervention in Syria than any other Western leader, was keen to stand with France and America. He recalled parliament from its summer recess to debate a British military response on August 29th, expecting to secure a majority in support of action. However the opposition Labour Party made it clear that it would not vote to authorise a strike until Mr Ban’s inspectors reported, and enough of Mr Cameron’s backbenchers are opposed to military action that he cannot get a motion through parliament without Labour support. If Britain is to participate in the attacks, they need to be delayed until after the weekend.

Some members of Congress would have liked to have been recalled from recess as their British counterparts were. They have grumbled about a lack of consultation, and fret openly about the war-weariness of their own constituents. Still, influential Republicans and Democrats have fallen in behind the new consensus.

Both within countries and between them, though, that consensus stretches only to a police action aimed to punish and deter. The motion before the British House of Commons notes that it “does not sanction any action in Syria with wider objectives”. The White House has repeatedly asserted that any actions in Syria are “not about regime change”.

This may sound strange. An attack on Mr Assad’s airbases, communication systems, and suchlike (see article) would seem, objectively, to aid the rebel cause. And it is official American policy that Syria’s future cannot include Mr Assad in power; it offers cautious, limited support to moderate factions within the patchwork of groups struggling to get rid of him.

Change is not gonna come

But the White House means something quite specific by “regime change”: the toppling of a dictator by force of arms, in a campaign led, and thus “owned”, by America. It is Mr Obama’s view that only a negotiated political settlement can end the killing, and that no neat military solutions exist. Indeed, military actions, including the one currently being planned, could easily make things worse.

Even if the White House did want to use military power in direct pursuit of regime change, the country has no stomach for adventurism. America is today, as it was before the invasion of Iraq, a place of flags, patriotic bumper stickers and public reverence for troops serving overseas. But it no longer has any patience for neoconservative notions of transplanting democracy into the Muslim world, and is distrustful of anything that conjures echoes of Iraq. Democrats, as well as Republicans, pushed for the swift release in declassified form of the intelligence that underlies the White House’s case, unwilling to take anything on trust.

Throughout the Syria crisis polling has shown large majorities of Americans opposed to intervention there—though that opposition has wavered when respondents were asked about a hypothetical use of chemical weapons. Much the same is true in Britain.

When America’s most senior military commander, General Martin Dempsey, recently told a congressman that he thought American force “can change the military balance, but it cannot resolve the underlying and historic ethnic, religious and tribal issues that are fuelling this conflict” he was speaking as one with his boss. Yet that boss, who has consistently asked for evidence that intervention will not make things worse, and when offered it has consistently been unconvinced, now finds himself on the brink of an intervention whose effects are unknowable.