A shepherd looks over a valley in Halabja, Iraq, that was the site of a chemical weapons attack in 1988. (Marco Di Lauro/Getty Images)

The Syrian military controls a large stockpile of the chemical weapon sarin, and for the duration of the country's now two-year conflict, the world has been watching warily for that deadly agent's release. Last week, the Obama administration revealed it had intelligence suggesting that some Syrians had been exposed to the chemical, although it's still not clear exactly what happened.

Sarin was first developed in Nazi Germany and later used by Saddam Hussein's forces against Iraqi Kurdish civilians. But what does it actually do to those exposed to it? The Atlantic's James Hamblin takes a look at the compound and its history. He also explains, in medical detail, how it can turn our own nervous system against us:

Sarin is unique in potency but not in mechanism. There are other drugs, pesticides, and plants that work the same way. They are called cholinesterase inhibitors.
Our nerves talk to each other by releasing chemicals called neurotransmitters. The amount of a particular neurotransmitter helps determine whether a nerve fires or not. What so-called nerve agents do is alter those neurotransmitters. They kink the signaling between our nerves, telling them to do things they normally do, but with altered frequency.
After a neurotransmitter has done its job, delivered its message, an enzyme usually comes along and demolishes it. But nerve agents block those enzymes. The enzyme can't break down the neurotransmitter, so the neurotransmitter stays around and keeps giving its message. If that message was, say, to release a little water onto your eye because your eye was dry, now the repeated message becomes "make your eyes water uncontrollably."

It gets worse from there:

Within seconds of exposure to sarin gas (or liquid, which evaporates easily), we start to notice the immediate effects of acetylcholine buildup. First, our smooth muscles and secretions go crazy. The nerves to those areas keep firing, keep telling them to go. The nose runs, the eyes cry, the mouth drools and vomits, and bowels and bladder evacuate themselves. It is not a dignified state.
Since sarin has no smell or taste, the person may very well have no idea what's going on. Their chest tightens, vision blurs. If the exposure was great enough, that can progress to convulsions, paralysis, and death within 1 to 10 minutes.

Sarin's horrific effects help explain why the Obama administration is so intent on holding a "red line" against its use. The taboo against chemical weapons has largely held for decades, one of the world's few successes in restricting war. A chemical war, because its tools are so much better suited for use against large numbers of civilians than for defeating an armed enemy, is, as I wrote recently, a qualitatively different kind of war.

Of course, the fighting in Syria has already claimed tens of thousands of lives, often in attacks that target civilians or use mortars and air strikes that kill indiscriminately. Is that just as bad as using a small amount of sarin, which is all it would take to cross the red line? Is it worse? It depends how you draw the line.