Recently, law enforcement officers have deployed chemical crowd control devices, such as tear gas and pepper spray, on protesters against racist injustice in cities across the United States. These technologies, which have generated controversy, are rooted in the military technology of World War I.

But as shocking as the images of gassed and incapacitated people coming out of Philadelphia and Portland, Ore. — and just this week, Atlanta — have been, the use of chemical weapons in civilian law enforcement is not new. In fact, law enforcement used similar weapons in similar ways as a response to the racial violence and labor unrest in the years that followed World War I. Although U.S. Army leadership feared using chemical weapons against civilians could be dangerous, it ultimately issued guidelines on their use in 1921. Revisiting this guidance may shed light on the deployment of tear gasses and other chemical weapons on Americans today.

The U.S. Army confronted the widespread use of chemical weapons for the first time in its history on the battlefields of World War I. The British, French and Germans began using poison gas in 1915, and the advent of chemical warfare required new innovations. Accordingly, the U.S. Army established the Chemical Warfare Service (CWS) to coordinate military activities related to chemical weapons. There, trained chemists and engineers worked to address the technological challenges of gas warfare. They discovered methods of military gas production, ways to deploy gasses in combat, developed improved gas masks and researched medical treatments for gas injuries.

Peacetime left the CWS searching for a mission, and the domestic social and political upheavals that followed the war offered one. Racial animosities exploded into violent confrontations, particularly during the Red Summer of 1919. In 1921, White residents attacked Black residents and destroyed homes and businesses in Tulsa, in an incident that remains one of the worst acts of racial violence in U.S. history. Post-World War I also saw the height of the Red Scare in early 20th century America, with instances of labor unrest across the country.

The head of the CWS, Gen. Amos A. Fries, argued tear gas and other chemicals classified as “irritants” could be used as crowd control devices to quell domestic unrest, but Army leadership opposed the plan. Among others, the secretary of war, Newton Baker, and the commander of the American Expeditionary Forces, Gen. John J. Pershing, knew how chemical weapons had been used on the battlefields of World War I, and they did not want them used against civilians in the United States. In 1919, Baker expressly prohibited using tear gas or other chemical agents for crowd control or providing local law enforcement with such chemical devices.