Secretary of State Hillary Clinton has stressed that, should Syrian President Bashar al-Assad in his desperation deploy some of his chemical weapons stockpile against his own people, he will cross "a red line for the United States." She warned, "suffice to say we are certainly planning to take action."
So why go to all the trouble of drawing a red line around chemical weapons? Why make such a big deal over them when Assad is already killing so many Syrians without them? I can't tell you what's happening inside Clinton's brain, or behind closed doors at the White House or State Department, but there is a long-established international norm against chemical weapons. And that norm has value well beyond this one conflict in this one country.
It would certainly be nice if we lived in a world where conventional weapons were never used or at least never used against civilians, and that's a goal worth aspiring to. But we live in a world where we still have to manage the conflicts we can't prevent. As long as war is a facet of human existence, it's worth upholding the norm that states do not use chemical weapons in those wars.
Chemical weapons were not always so taboo. The norm against their use was first established by the 1925 Geneva Protocol, which came at the enormous cost of World War I. That treaty and the norm it enforced didn't prevent chemical weapons from being used during World War II, but it did perhaps contribute to their absence from European fronts and to Japan's decision to use them more sparingly than they likely would have otherwise. World War II was still awful, but it was considerably less awful than if chemical weapons had been used as widely as they were in the previous world war.
Syria's violence is likewise still terrible even without chemical weapons, but it is less terrible than it would almost certainly be if the state felt it could freely deploy its vast chemical weapons. And, as long as there are conflicts involving states that possess or have access to chemical weapons, those conflicts will be less deadly if the chemical weapons remain locked up.
The U.S. record in enforcing the norm against chemical weapons is not perfect. During the brutal Iran-Iraq war of the 1980s, in which the United States sometimes backed Saddam Hussein's effort against a mutual Iranian enemy, Iraq's use of chemical weapons was at times overlooked. The history of what exactly the United States knew about Saddam's chemical attacks as they happened and why it chose to respond (or, more accurately, not respond) as it did is still shrouded in some mystery. But an investigation by Joost Hiltermann of Human Rights Watch concluded that the United States may have played down the reports, or at least avoided calling attention to them. Though Saddam of course fell many years later, he suffered relatively little at the time for his decision to use chemical weapons.
According to a Foreign Affairs review of Hiltermann's book on the Iraqi gas attacks, "the fallout of these developments has been an enhanced readiness among states to stock and prepare to use weapons of mass destruction [and] an Iran set on never again being without such weapons." Whether or not that's an accurate characterization of countries' motivation in amassing chemical weapons or other weapons of mass destruction, it highlights the potentially wide-reaching and long-term risks of even a single incident of chemical weapons usage. That risk alone underscores the importance of the international norm against chemical weapons, and informs why the United States is so insistent on upholding it.