Like many others, I have been sedulously following the events related to the use of chemical weapons in Syria. In August 2012, when Barack Obama said the use of chemical weapons represented a “red line” for American involvement, there had been fewer than 25,000 deaths in the civil war, according to the Center for Documentation of Violations in Syria. One year later, the estimate, which is low compared to other groups, is approaching 75,000. (The U.N., for instance, puts the total number of casualties at over 100,000.)
It was the manner of the deaths of approximately 1,000 people two weeks ago which met the conditions for Obama's red line. Obama hasn't made clear what sort of response will occur—he has now decided to take the issue before Congress—but he has been clear that the use of chemical weapons is morally wrong, not just politically destabilizing or illegal.
What is it, exactly, that explains the special wrongness of chemical weapons? John Mueller argues in a Foreign Affairs article called “Erase the Red Line” that chemical weapons are seen as a special breed of villainy due to First World War propaganda. The British were trying to persuade America to join the war, so they inflated death tolls from German chemical weapons attacks, while in reality chemical weapons accounted for fewer than one percent of total war fatalities, and only two or three percent of those gassed actually died.
But appealing to numbers misses the point Obama and others are making regarding chemical weapons. After all, up to 100,000 people died by non-poison means, and while those acts have been condemned, they haven't brought America close to military action.
In The Better Angels of Our Nature, Steven Pinker describes how despite large stockpiles of poison gas, no European country used it in battle during the Second World War. This is true even of the Nazis, who had no qualms about using it to commit genocide. Such weapons have been used only a few times since, most notably by Saddam Hussein against Iranians and Iraq's own Kurdish population during the Iran-Iraq War.
Pinker says our psychological reaction to chemical weapons lies in our repugnance of poison generally. To poison someone is to commit an act of trickery or cowardice, whereas fighting with other means is noble, brave, and valorous. Chemical weapons allow no chance of fighting back, which is also why some argue killing with drones is immoral.
For those of us who put little stock in virtue ethics, it's harder to see what the difference is. Of course it's a horrific thing to die of asphyxiation, but it's not clearly worse—to me anyway—than dying from a bullet or shrapnel. It seems no moral improvement if we suppose that Hussein had killed thousands of Kurds by traditional bombing. Also, many think it's acceptable to kill in other cases in which the victim is unable to retaliate, such as with snipers. (Perhaps intuition differs there because snipers possess skill, and it is one person killing another person.)
There are, of course, better and worse ways to die, even in war. But what should matter is how many people die, and the pain they suffer in the process. Chemical weapons as a category don't change the balance of that calculation.