Aaron Stein, who studies the Middle East and arms control at the Atlantic Council, a research group, said that for a leader like Mr. Assad, who appears unconcerned with civilian death, chemical weapons presented a way to solve his battlefield problems.
“They are effective weapons,” he said, that were best suited to capitalize on Mr. Assad’s air power superiority and compensate for his manpower shortages.
The chemical weapons attack last week, in Syria’s northwestern Idlib Province, illustrated the ghastly utility of such weapons. Rebels were advancing there, threatening Mr. Assad’s hold.
Unable to retake that territory easily with conventional means, Syrian forces appear to have used chemical weapons both on and behind the front lines, simultaneously halting the opposition’s advance while devastating its supply lines and staging areas. It also forced rebels into concentrated areas where they could be killed in large numbers.
Last week and in 2013, chemical weapons appear to have addressed a desperate battlefield problem for Mr. Assad. Because no other tool can so reliably solve that problem, his calculus has proved difficult to change.
Controlling the Population
Wars are ultimately won not just on the battlefield but also through the allegiances of the population, whose support is crucial for maintaining any fight and for controlling territory.
This has been a central struggle for Mr. Assad, who faces a seemingly unsolvable problem of his own making. After years of atrocities against civilians, particularly those in strategically crucial areas such as Idlib, much of the population has little reason ever to back his government.
Unable to win popular support, he has embraced a strategy not unlike that of the Islamic State: to terrorize civilians to such an extent that they will drop any support for opposition groups who have been proved unable to protect them.
In this atrocity arms race, chemical weapons have a psychological power far exceeding even conventional means of mass killings, like barrel bombs. A medical study published in 2006 found that Iranian survivors of chemical weapons attacks suffered lifelong post-traumatic stress whose severity exceeded that caused even by front-line combat.
Chemical weapons, in this way, serve a political function similar to that of their battlefield role: denying the opposition a victory.
The Removal Problem
Even sophisticated chemical weapons are relatively easy to hide and clandestinely transport. Inspections or other forms of monitoring can track large-scale production facilities, but they cannot watch every shed or hole in the ground.
This means that even leaders who say they have surrendered their chemical weapons — as Mr. Assad did in 2013 — can hide small amounts without fear of discovery. Col. Muammar el-Qaddafi of Libya, despite a similar deal, hid chemical weapons around the country.
So did Saddam Hussein, the former Iraqi leader. American troops who discovered the caches decades later found that the weapons, though poorly stored, were still deadly.
For leaders like Mr. Assad, who rightly fears armed uprising (as was the case with Colonel Qaddafi or Mr. Hussein), the perceived upside of retaining a secret emergency stockpile far exceeds any potential costs.
A small reserve does not serve the same utility as a full arsenal, analysts emphasize. Mr. Assad, in surrendering the vast majority of his stockpile and production, has forever lost the ability to deploy them extensively in a large-scale war, for example against Israel.
But it is functionally impossible to remove every last canister without also resolving a leader’s underlying reasons for desiring the weapons, meaning Mr. Assad will most likely permanently retain the ability to launch small and infrequent but devastating strikes like last week’s.
Lack of Leverage
In deterring chemical attacks in Syria, the world faces a severe version of a problem it faces more broadly there: lack of leverage.
In any attempt to change a government’s calculus, the crucial challenge is to impose costs exceeding whatever benefit the government gains from its existing behavior.
If Mr. Assad views chemical weapons as important to both his battlefield and political strategies, then an effective deterrent must alter both.
Mr. Assad is aided by Russian forces who are so well integrated into the war that any deeply damaging strikes would risk killing Russians, potentially creating a wider conflict between the nuclear powers. Russia also shields Mr. Assad diplomatically.
But even Moscow has shown that it, like every great power trying to manage a smaller ally, has limited influence over its proxy. And Russia is not Mr. Assad’s only sponsor: He also relies on heavy support from Iran.
Iranian and Russian aims in the war differ in ways that reduce each other’s influence and could enable chemical weapons use, regardless of whether those governments want such an outcome.
Moscow’s goal appears to be preserving Mr. Assad’s government — a strategy that could ultimately require a political settlement, which becomes harder to achieve when chemical attacks galvanize the world against Mr. Assad. Tehran, meanwhile, appears to be pushing for total victory over the opposition, which favors chemical weapons use.
Those crosscutting alliances give Mr. Assad an incentive to use chemical weapons while weakening his sponsors’ ability to constrain him.
A Weakened Norm
The international norm against chemical weapons “has never really taken hold in the Middle East,” Mr. Stein said.
Middle Eastern governments stand out for not just stockpiling but also using chemical weapons, though not against Israel or outside forces.
Egypt used chemical weapons in Yemen in the 1960s, when it intervened in that country’s civil war. Iraq used them domestically and against Iran, both in the 1980s. The government of Chad accused Libyan forces of using chemical weapons during their war in the 1980s, though this claim remains contentious.
Great power politics played a role: The Soviet Union helped build Egypt’s chemical weapons program and tolerated their use in Yemen; the United States tolerated Iraq’s use of them against Iran, their mutual enemy.
Israel’s nuclear program also created a strategic imbalance in the region. Chemical weapons give Arab states a passable deterrent, which the international community appears to have tolerated as a means of restoring that balance.
The weakened norm emboldens Mr. Assad’s use of chemical weapons for two reasons. First, his commanders face less of a taboo in deploying them and so are less likely to object. And, second, Mr. Assad has little to gain in upholding a norm that never worked to his benefit.
States uphold norms not just for fear of outside punishment but also out of a hope that the norm will constrain potential adversaries. But Mr. Assad has scant reason to hope that other Arab states or actors will abide by a norm that, for them, hardly exists.
All or Nothing
In waging total war against rebels and his own population, Mr. Assad has backed himself into an all-or-nothing conflict. Even if chemical weapons play only a secondary role in his political and battlefield strategies, he faces margins too slim and stakes too high to abandon them without a fundamental shift.
More limited action, such as the Obama administration’s disarmament efforts or the Trump administration’s missile strikes, could mean that “in the near term, Bashar will think twice about using chemical weapons,” Mr. Stein said.
Longer term, such moves are unlikely to “alter the dynamic of the conflict” that drives the use of chemical weapons.
A round of missile strikes, Mr. Stein added, may “make everybody feel better, but it ultimately doesn’t change the status quo.” As with past efforts, unless something can shift those deeper dynamics, he said, “we just kicked the can down the road.”
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