The Bonus Army was a group of 43,000 demonstrators 17,000 veterans of the United States in World War I, their families, and affiliated groups who gathered in Washington, D.C., in mid-1932 to demand early cash redemption of their service bonus certificates. Organizers called the demonstrators the "Bonus Expeditionary Force" (B.E.F.), to echo the name of World War I's American Expeditionary Forces, while the media referred to them as the "Bonus Army" or "Bonus Marchers". The demonstrators were led by Walter W. Waters, a former sergeant.

Many of the war veterans had been out of work since the beginning of the Great Depression. The World War Adjusted Compensation Act of 1924 had awarded them bonuses in the form of certificates they could not redeem until 1945. Each certificate, issued to a qualified veteran soldier, bore a face value equal to the soldier's promised payment with compound interest. The principal demand of the Bonus Army was the immediate cash payment of their certificates.

On July 28, 1932, U.S. Attorney General William D. Mitchell ordered the veterans removed from all government property. Washington police met with resistance, shot at the protestors, and 2 veterans were wounded and later died. President Herbert Hoover then ordered the U.S. Army to clear the marchers' campsite. Army Chief of Staff General Douglas MacArthur commanded a contingent of infantry and cavalry, supported by six tanks. The Bonus Army marchers with their wives and children were driven out, and their shelters and belongings burned.

A second, smaller Bonus March in 1933 at the start of the Roosevelt administration was defused in May with an offer of jobs with the Civilian Conservation Corps at Fort Hunt, Virginia, which most of the group accepted. Those who chose not to work for the CCC by the May 22 deadline were given transportation home. In 1936, Congress overrode President Roosevelt's veto and paid the veterans their bonus nine years early.

Tear gas, also known as a lachrymator agent or lachrymator (from the Latin lacrima meaning "tear"), sometimes colloquially known as "mace" after an early commercial aerosol, is a chemical weapon that stimulates the nerves of the lacrimal gland in the eye to produce tears. In addition, it can cause severe eye and respiratory pain, skin irritation, bleeding, and blindness. Common lachrymators both currently and formerly used as tear gas include pepper spray (OC gas), PAVA spray (nonivamide), CS gas, CR gas, CN gas (phenacyl chloride), bromoacetone, xylyl bromide and Mace (a branded mixture).

While lachrymatory agents are commonly deployed for riot control by law enforcement and military personnel, its use in warfare is prohibited by various international treaties. During World War I, increasingly toxic and deadly lachrymatory agents were used.

The short and long-term effects of tear gas are not well studied. The published peer-reviewed literature consists of lower quality evidence that does not establish causality. More rigorous research is needed. Exposure to tear gas agents may produce numerous short-term and long-term health effects, including development of respiratory illnesses, severe eye injuries and diseases (such as traumatic optic neuropathy, keratitis, glaucoma, and cataracts), dermatitis, damage of cardiovascular and gastrointestinal systems, and death, especially in cases with exposure to high concentrations of tear gas or application of the tear gases in enclosed spaces.