Second, there was the "indiscriminate" impact of gas warfare. It was diffused broadly in the atmosphere—and could blow back into the offensive users or affect civilian populations. This uncontrolled aspect of gas warfare led to opposition among some military leaders on all sides.

Finally, there was a fear of an unknown future. Despite the relatively small number of actual deaths and casualties from chemical warfare compared to the horrific total, there was worry about its much broader use in the future. The inhuman, terrifying images of soldiers in gas masks fed these emotional concerns.

Together, these reasons led to a special strain of public fear and loathing that prompted the collective action embodied in the 1925 protocol. It stated that such warfare "has been justly condemned by the general opinion of the civilized world." Forty nations originally agreed to the protocol. Today that number is more than 130—although the United States did not officially adopt the protocol until 1975. And Syria adopted it in 1968.

The exceptional nature of chemical and biological warfare was reflected in the Chemical Weapons Convention, which entered into force in 1997. It sought to remedy many of the defects of the Geneva Protocol by prohibiting manufacturing and stockpiling, requiring destruction of existing stocks, establishing a verification system, and establishing a special monitoring body, the Organization for the Prohibition of Chemical Weapons. More than 180 nations have ratified the CWC—but not Syria.

Of course, the conventions by themselves do not have an enforcement mechanism. The legal route to collective action against miscreant states today requires invocation of general UN legal process. And there have been numerous other instances of chemical weapon use before Syria—from Japan against China in the early 40s to Iraq against Iran and the Kurds in the 80s.

Yet, at least in theory and often (though not always) in practice, the world community, going back at least to the 1925 protocol, has made a decision that chemical and biological weapons are morally different from conventional weapons--which, some would say, can kill just as widely, horribly and indiscriminately.

Today, of course, chemical and biological agents are classified as weapons of mass destruction. And the reasons advanced in the 1920s are much the same as advanced now by the Obama Administration in its argument to take action in Syria (although the specter of asymmetrical use by terrorists is added to use by states). Secretary Kerry has said that the deaths of more than 1,400 Syrian civilians because of chemical agents shows "the large-scale, indiscriminate use of weapons that the civilized world has long ago [agreed] must never be used." He added, "Our sense of basic humanity is offended…by this cowardly crime" involving the "world's most heinous weapons." The administration is showing films of the deaths that, per CNN, reveal "Men sprawled on a tile floor, shirtless and convulsing. Children, too, seemingly unable to control their shaking and flailing. Panic and screams in the background."