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.

Yet Fries worked tirelessly the next two years to try to persuade others in the military and government that chemical weapons were effective, and even humane, alternatives to other riot control measures. Part of his motivation stemmed from the predicament facing the CWS. Before World War I, Fries, a career officer, served in the Corps of Engineers in the Philippines under Pershing before returning to the United States, where he directed engineering projects in Southern California and Yellowstone National Park. Ambitious, nationalistic and outspokenly anti-communist, Fries devoted his post-World War I career to making the CWS a permanent part of the U.S. military during the relatively lean postwar budget years.

To accomplish this goal, he worked to demonstrate peacetime uses of chemical weapons, including for law enforcement. His efforts included flamboyant public relations events to make tear gas seem relatively harmless. In August 1921, Fries organized a news event where he tear-gassed a group of 60 Girl Scouts, including one of his own daughters, during their visit to Edgewood Arsenal in Maryland.

Fries’s efforts found an audience when at the same time as his campaign, the Army became involved in a large mine workers’ strike in West Virginia. The War Department relaxed its previous opposition to tear gas and other crowd control chemicals, and it made CWS officers and tear gas weapons available for use during the operation.

While the unrest in the Blair Mountain region was ultimately resolved without gas, a sea change was taking place within the federal government. Party power had shifted in Washington, D.C., with the inauguration of President Warren G. Harding in March 1921. His secretary of war, John W. Weeks, did not share his predecessor’s opposition to chemical warfare. While some in the military and in Congress remained opposed to chemical weapons in general, and diplomats debated prohibitions against chemical warfare internationally, Fries’s efforts were convincing enough policymakers that the CWS’s work, including tear gas, should continue.

In this context, the CWS offered guidelines in a first-of-its-kind memorandum titled “Provisional Instructions for the Control of Mobs by Chemical Warfare.” This document, issued Nov. 21, 1921, offers a glimpse at the intended uses of chemical weapons as crowd control measures after World War I — and gives insight into how law enforcement conceived of unrest at home in terms of war.

“Provisional Instructions” classified three types of “mobs”: an armed, disciplined group of “500 up to a few thousand” members; a smaller group with fewer armaments and less organization; and a group composed of men, “women, boys, and sometimes small children, unorganized, but excited and irresponsible.” The document recommended different chemical devices for use against these different groups — including tear gas and white phosphorus grenades, 4-inch mortar rounds and aviation bombs. The manual even drew comparisons to the poisonous gasses used against enemy combatants in the war. “Provisional Instructions” stated common knowledge about the effects of war gasses “tends to increase the fear of these weapons,” making them more effective at suppressing crowds.

The guidelines advised those using tear gas — whether military officers or local police — to create avenues of escape to minimize trampling in large groups and to make special considerations if using gas in enclosed spaces, because so-called nonlethal gasses can become lethal in higher concentrations. The guidelines cautioned against firing or dropping these weapons directly into crowds, because devices such as tear gas grenades “will seriously injure anyone it hits in its flight.” “A white phosphorus smoke bomb should never be shot into a mob,” the document reads, “unless conditions have become so grave as to require treating the mob as one would treat enemy troops.”

It anticipated detailed operational plans would be made before crowd control devices were deployed, treating the use of such chemical agents as less of a law enforcement, and more of a battlefield, measure. The guidelines also detailed two “tactical problems,” portrayed as hypothetical but bearing close resemblance to recent events. One simulated using tear gas in a mine labor dispute, not unlike the one in 1921 in West Virginia, and one simulated using tear gas at an urban riot in a fictitious “Brownton, Texas,” not unlike Tulsa.

The publication of these guidelines reflected the belief that tear gas and other crowd-control devices would be effective fear-inducing weapons against organized laborers and minority groups. This perceived effectiveness outweighed their known risks and harms by 1921. Municipal law enforcement and National Guard units across the country contacted Fries and the CWS for information and training, and the weapons began to be used soon afterward.

The Kansas National Guard procured tear gas for use against mine laborers and their families during a demonstration in December 1921, but the strike ended before the gas was used. The following June, law enforcement in Jackson, Mich., fired tear gas at a large crowd attempting to lynch a prisoner at the county jail. Tear gas was used in more routine law enforcement as well. In Chicago in 1923, for example, when police threw tear gas bombs through a transom into a hotel room to apprehend an armed individual, hotel guests and the police themselves were also affected by the gas. In 1932, the U.S. Army used tear gas against civilians when it was ordered by President Herbert Hoover to disperse the Bonus Army demonstrators in Washington, D.C.

As we debate critical questions about racial injustice, police militarization and the use of chemical agents against protesters, proposals for reform must be considered in light of the origins of the policies they seek to address. After returning from the trenches of World War I, the officers of the U.S. Army Chemical Warfare Service applied their wartime expertise to civilian law enforcement, and the ongoing use of chemical weapons against group demonstrations remains a terrible consequence of that decision.

Disclaimer: The views expressed here are the author’s and not necessarily those of the Department of State or the U.S. Government. The author is grateful to Duane B. Miller for providing a digital copy of “Provisional Instructions for the Control of Mobs by Chemical Warfare” from the collections of the U.S. Army Heritage and Education Center in Carlisle, Pa.