But rioting did not spur the earliest military use of CS in South Vietnam, in combined operations of U.S. and local forces. Instead, the chemical was released in situations where guerrillas and U.S. personnel were intermingled and conventional weapons might kill both. Soon, however, the most common use of CS came not in the setting of mingled friend and foe, nor even in riot control.
Instead, CS was used for two primary combat purposes. One was to “flush” people out of hiding. A key challenge for the United States during its war in Vietnam was simply locating the “enemy.” CS, whose effects were so unpleasant, was supposed to force people out of hiding. Whether in heavy undergrowth or in tunnels and bunkers, the U.S. military used CS to force people out into the open. In many cases, they were then killed with airstrikes, artillery, or conventional gunfire. CS made targeting otherwise hidden people easier. The second purpose was to control access to terrain. New formulations of CS were developed that could be persistent and even water-resistant, meaning they could contaminate buildings, bunkers, tunnels, and landscapes for weeks or longer. Or, to keep people away from a given location, CS would be used to create a barrier. We should keep this legacy in mind today, as law-enforcement agencies on borders from Ceuta to Gaza to San Diego deploy tear gas.
Surprisingly, the U.S. military no longer uses CS in combat operations due to regulations and international law. These resulted from public outcry against CS, which only intensified after news reports initially revealed its use in South Vietnam.
In 1969, President Richard Nixon triangulated among the antiwar movement, critics in Congress, and the Soviet Union by committing to a policy of no first use of chemical weapons. But CS was excluded. Six years later, however, President Gerald Ford responded to antiwar sentiment social movements, scientists, and Congress expressed by issuing an executive order banning the military use of “riot control agents” and herbicides in combat, except under special circumstances. The 1993 Chemical Warfare Convention superseded the Geneva Protocol to outlaw chemical weapons in war, and the United States ratified it in 1997. This agreement expressly did not cover domestic law-enforcement use of “riot control agents.” As a result, the U.S. military would continue to uphold Ford’s executive order while civilian law enforcement remained unfettered. This legal situation conditioned contemporary deployments, from Ferguson, Missouri, to San Diego, California.