SANA / handout / REUTERS Soldiers loyal to Syria's President Bashar al-Assad enter part of Jobar neighbourhood in Damascus, which had been controlled by fighters of the Free Syrian Army, June 3, 2013.

With Western leaders weighing whether to intervene in the Syrian war with a Libya-style bombing campaign or by arming the Syrian rebels with heavy weaponry, it would seem logical that President Bashar Assad would try to avoid provoking the West into agreeing on a coordinated assault against his forces. But if there’s one thing more likely than any other to push the West into more direct military action it’s the use of chemical weapons. On May 4 the French and British governments announced that they have evidence that the nerve agent sarin has been used in Syria; both governments believe it is Assad’s forces, rather than the rebels, who have used the weapons. The as-yet unproven claims are the latest in a line of similar allegations that Assad has has used chemical weapons in the war. Which raises a question: Why would a regime that is already facing broad condemnation around the world, and which has ample supplies of conventional weapons, so risk provoking a Western-led intervention?

First, the facts, or as factual as claims can be in the devastating conflict, which some estimate has killed about 90,000 people since February 2011. On Monday, the U.N. panel charged with investigating Syria’s hostilities issued a report in Geneva, saying that there was evidence that “limited quantities of toxic chemicals” had been used in four attacks in March and April, twice in Aleppo, once in Damascus, and once in Idlib. Paulo Pinheiro, the Brazilian diplomat who heads the four-person group, told reporters that although investigators had “reasonable grounds to believe” chemical weapons had been used, they did not have absolute proof which chemicals had been deployed, or whether the rebels or the regime had used them. White House officials said on Tuesday that the U.S. needed more evidence before drawing firm conclusions. Yet in Paris, French Foreign Minister Laurent Fabius told France 2 Television that in one case the chemical was certainly sarin gas, a potentially lethal nerve agent, and that there was “no doubt that the regime and its accomplices” were the culprits, based on data from a French laboratory which had tested victims. France passed the results to a U.N. team investigating chemical weapons use in Syria. “We have no doubt that the gas is being used,” Fabius said. “The laboratory tests are clear.”

But why would Assad want to use them at all? Chemical weapons, at least in small quantities, would seem redundant in the conflict, since Assad’s jets have effectively pummeled rebel areas for more than two years, inflicting crippling losses and ravaging entire cities. In addition, Syria’s closest international ally, Russia, warned Assad strongly against chemical attacks soon after the conflict began; Moscow is a signatory to the 1993 Chemical Weapons Convention, which outlaws them, and has destroyed much of its own stockpile. “Russia was the country which had the most impact on Assad in terms of whether he would use chemical weapons or not,” says Dina Esfandiary, research associate in the nonproliferation and disarmament program at London’s International Institute for Strategic Studies (IISS). “They said it was absolutely not okay.” Esfandiary says she believes Assad might be “testing the international community to see what he can get away with.”

For months, Western leaders have fretted over Syria’s chemical stockpiles, which the IISS believes run to hundreds of tons, and include mustard gas and sarin gas. They fear the weapons could be transferred to Assad’s ally Hizballah in Lebanon, or that they might fall into the hands of Syria’s more extremist rebel groups. There is also the possibility that Assad could choose to unleash them on thousands of civilians, much like Iraq’s Saddam Hussein did in 1988, when he dropped bombs containing chemical weapons on Kurdish villages around the northern city of Halabja, killing about 5,000 people and prompting world leaders to ban the weapons and destroy their stockpiles.

The worry is that chemical weapons might be used in Syria’s end game, if the regime believes it has nothing to lose, or if it decides that the West will intervene anyway. The U.N. report came a week after EU leaders let an arms embargo against Syria lapse, allowing individual European countries to begin arming rebel groups. And President Obama, who has so far rejected any prospect of military intervention in Syria, has said the use of chemical weapons would be “a red line for us” that would “change my calculus.”

Given the suggestion that the U.S. might intervene if it believes chemical weapons are being used, Assad would seem to have little gain by deploying them now, especially at a time when his forces are gaining ground. Perhaps because it makes little obvious sense, U.S. and British officials seem reluctant to rush to judgment, especially since false information about Saddam’s weapons of mass destruction led the U.S. and Britain to invade Iraq in 2003. In fact, Saddam had none.

Syria’s stocks of chemical weapons seem more real than that. But that does not mean that Assad intends to use them in a massive attack, or has done so. “Although everyone is pointing the finger at him it is very hard to find hard, definite proof that it was him,” Esfandiary says. “It could be a rogue general, or a rogue canister.”

One clue about Assad’s motivations came last month, when a Syrian chemist who said he had helped design Assad’s wartime chemical weapons program told Al Jazeera that the sarin attacks in March and April were intended to “incapacitate rebels and force them out of strategic areas, while keeping the deaths among their ranks limited.” He claimed that Syria’s chemical stockpile included 700 tons of sarin, plus what he described as 3,000 bombs capable of being filled with deadly chemicals, and more than 100 chemical warheads for Scud missiles, suggesting that the warheads had already been readied for use.

Al Jazeera did not name the chemist, who it said fled Syria last December, and there is little way of verifying his information. So far, there is little sign of Assad choosing that go-for-broke option, a tactic that the chemist said the regime would deploy only if it “no longer cares about the world knowing.”

But Assad does not seem to have reached that point, judging by the small scale of the attacks that suggested the use of chemical weapons. Rocket attacks on March 19, in parts of Aleppo and Damascus, killed only 25 people. Both Syrian and Russian officials blamed rebel groups for using chemical weapons that day, while the opposition said the regime had done so, and that the victims had difficulty breathing and that their skin had turned blue. And although Obama has made chemical weapons the “red line” for intervention, the wide-scale killing of civilians has already deeply shaken world leaders. “The reason chemical weapons are such a no-no is that there is a stigma attached to them, and a convention banned them,” Esfandiary told TIME on Tuesday. “Plus, it looks terrifying, and has a big psychological effect.”

In fact, under the terms of that international treaty, there is no legal obligation for the world to intervene to stop their use. That is different, for example, to the circumstances that led NATO to launch its bombing campaign against Muammar Gaddafi’s forces in Libya in March 2011. That campaign began after the U.N. voted that Gaddafi threatened the lives of civilians in Benghazi, triggering member states to protect them.

The chemical weapons ban has had success, however. An international program charged with implementing the treaty has destroyed about 80% of the world’s stockpiles, including much of those in the U.S. and Russia. Yet Syria, along with North Korea, remains a holdout, among just a handful of countries that have never signed the chemical weapons ban. And that fact that looks set to remain so long as Assad clings to power.