The smell of pure sarin gas is the smell of nothing at all, and also the smell of death. Sarin is a nerve agent, and just a whiff will disturb virtually every bodily function. The nose begins to run uncontrollably. Pupils constrict, vision clouds over, and the mouth froths. Chemical warfare expert Tim Blades, who was accidentally exposed to sarin as a U.S. government employee,​ likens the feeling to having “something big and rotten stuck inside [the] abdomen.” Blades survived the exposure only because he received expert medical attention. If he had been exposed in a war zone, far from modern hospitals, he might have ended up like hundreds of Syrians — dead within hours.

In his new book, “Red Line: The Unraveling of Syria and America’s Race to Destroy the Most Dangerous Arsenal in the World,” The Washington Post’s Joby Warrick tells the story of President Bashar al-Assad’s chemical weapons program, from the dormant period when it was holstered in vats in facilities scattered around the country, to its use against civilians, to its supposed dismantling. Pressured by his ally Russia, Assad pledged to surrender his sarin and did in fact let the United States neutralize tens of thousands of gallons of it — enough to kill every Syrian many times over. But he kept a secret stash somewhere, and he probably still has it.

Stories about chemical and biological weapons are often oversold, for the same reason stories of cannibalistic serial killers are: Peculiar forms of murder repulse and excite us, and old-fashioned forms do not. (Most of Assad’s victims have died from conventional weapons, such as barrel bombs.) Chemical weapons are an extremely impractical and roundabout way to commit mass murder. A surprise gust of wind can render them harmless (or blow them back toward you); careless chemists tend to poison themselves in the production process. In World War I it took about a ton of mustard gas to kill a single enemy soldier. Iraq gassed 27,000 Iranian soldiers in the Iran-Iraq War but killed only 262 of them. By contrast a bullet to the brain works almost every time.

The Syrian civil war has killed about half a million people, and only a few thousand of them with chemical weapons. More than 1,000 died in a single sarin attack in Eastern Ghouta, near Damascus, in August 2013, and nearly 100 died in another sarin attack in Khan Sheikhun in April 2017. An undetermined number died from attacks using chlorine gas, which is much less toxic than sarin. The perpetrator of all these attacks, according to every reputable source, was the armed forces of Syria, loyal to Assad.

Warrick maintains a sense of proportion. He devotes short sections to efforts by the Islamic State to develop chemical weapons. But the Islamic State didn’t need these weapons to be fearsome; its militants could just run over people with trucks. The group’s chemical weapons program was never more than primitive and probably wasted its resources.

Overwhelmingly, Warrick’s emphasis is where it should be, on Assad, for whom chemical weapons were a highly developed and strategic program of terror. “Syrians died every day from bullets, blast wounds, and shrapnel injuries,” Warrick writes, “but to exterminate human beings with chemicals, as though they were fleas and cockroaches” — this was “a different order of savagery.” Lacking any legitimate military purpose, Assad’s chemical weapons existed to terrorize civilian populations by killing as indiscriminately as possible. Eliminating his arsenal was therefore a top international priority.

The efforts toward this goal were at times ingenious, even heroic. The unlikely protagonist of the first section is Ake Sellström, a Swedish chemical weapons researcher who led an international investigation of the Eastern Ghouta massacre and proved the Syrian government’s culpability. Surviving Syrians on the receiving end of the chemical attacks, such as a young doctor named Houssam Alnahhas, act with no less bravery by sharing advice with one another, even while being hunted by the government, on how to treat the victims and preserve evidence of their causes of death.

Blades, the American who survived sarin exposure, is another hero. Assad’s sarin is what is known as a binary chemical weapon — separated into two stable chemicals that become sarin when mixed. (Think of peanut butter and jelly, each kept in its own jar until ready to be commingled on sandwich bread.) Blades was tasked with destroying these chemicals after Assad surrendered them. He invented a portable contraption (known as the “Margarita Machine”) that converts burbling sarin precursors into a “broth the color of watery blue Kool-Aid,” roughly as harmful as the cleaning products under my kitchen sink. For weeks, Blades’s team operated these machines aboard a ship in the Mediterranean, rendering inert enough Syrian sarin to kill tens of millions of people (or fish, had the ship capsized). Toxic-waste disposal is not, on its surface, the most promising subject matter for a nonfiction thriller, but Warrick presents it sharply and compellingly.

Nonetheless it appears in the end that these efforts, however heroic, achieved almost nothing. Warrick rightly blames the Russians for guaranteeing Assad’s survival. They claimed to want a chemical-weapon-free Syria, but they slow-rolled every eradication effort and ultimately intervened on Assad’s side militarily, even as he continued to gas civilians. Warrick shies away from stating what his evidence strongly suggests: Assad’s chemical weapons program worked very well indeed, not by killing people but by giving Assad a bargaining chip and giving the Russians one more issue over which to burn time and energy negotiating. President Barack Obama’s famous comment about Assad’s chemical weapons — that using them would mean crossing a “red line” and triggering an American response — turned out to be nearly the opposite of true. Using chemical weapons did not end Assad’s ability to negotiate. It strengthened his position.

Did Assad want Sellström to tell the world that Syria had sarin and had used it? A secret chemical weapons capability would not have done Assad much good. But a known one could distract the international community while the Syrian civil war was won by conventional means. That is what appears to have happened. Warrick quotes then-Secretary of State John Kerry in conversation with a Russian counterpart: “If [the Syrian rebels] are truly bad guys, I don’t care how you kill them,” he says. “Just don’t use chemical weapons!”

Assad gassed just enough people to confirm to the world that he had no scruples — and that in exchange for being allowed to survive, he might refrain from doing so again. This is a diabolical way to run a country but a very effective way to stay in power.

Red Line

The Unraveling of Syria and America’s Race to Destroy the Most Dangerous Arsenal in the World

By Joby Warrick

Doubleday. 346 pp. $29.95