On April 22, 1915, German forces shock Allied soldiers along the western front by firing more than 150 tons of lethal chlorine gas against two French colonial divisions at Ypres, Belgium. This was the first major gas attack by the Germans, and it devastated the Allied line.

Toxic smoke has been used occasionally in warfare since ancient times, and in 1912 the French used small amounts of tear gas in police operations. At the outbreak of World War I, the Germans began actively to develop chemical weapons. In October 1914, the Germans placed some small tear-gas canisters in shells that were fired at Neuve Chapelle, France, but Allied troops were not exposed. In January 1915, the Germans fired shells loaded with xylyl bromide, a more lethal gas, at Russian troops at Bolimov on the eastern front. Because of the wintry cold, most of the gas froze, but the Russians nonetheless reported more than 1,000 killed as a result of the new weapon.

READ MORE: Tech Developments of World War I

On April 22, 1915, the Germans launched their first and only offensive of the year. Known as the Second Battle of Ypres, the offensive began with the usual artillery bombardment of the enemy’s line. When the shelling died down, the Allied defenders waited for the first wave of German attack troops but instead were thrown into panic when chlorine gas wafted across no-man’s land and down into their trenches. The Germans targeted four miles of the front with the wind-blown poison gas and decimated two divisions of French and Algerian colonial troops. The Allied line was breached, but the Germans, perhaps as shocked as the Allies by the devastating effects of the poison gas, failed to take full advantage, and the Allies held most of their positions.

A second gas attack, against a Canadian division, on April 24, pushed the Allies further back, and by May they had retreated to the town of Ypres. The Second Battle of Ypres ended on May 25, with insignificant gains for the Germans. The introduction of poison gas, however, would have great significance in World War I.

Immediately after the German gas attack at Ypres, France and Britain began developing their own chemical weapons and gas masks. With the Germans taking the lead, an extensive number of projectiles filled with deadly substances polluted the trenches of World War I. Mustard gas, introduced by the Germans in 1917, blistered the skin, eyes, and lungs, and killed thousands. Military strategists defended the use of poison gas by saying it reduced the enemy’s ability to respond and thus saved lives in offensives. In reality, defenses against poison gas usually kept pace with offensive developments, and both sides employed sophisticated gas masks and protective clothing that essentially negated the strategic importance of chemical weapons.

The United States, which entered World War I in 1917, also developed and used chemical weapons. Future president Harry S. Truman was the captain of a U.S. field artillery unit that fired poison gas against the Germans in 1918. In all, more than 100,000 tons of chemical weapons agents were used in World War I, some 500,000 troops were injured, and almost 30,000 died, including 2,000 Americans.

In the years following World War I, Britain, France, and Spain used chemical weapons in various colonial struggles, despite mounting international criticism of chemical warfare. In 1925, the Geneva Protocol of 1925 banned the use of chemical weapons in war but did not outlaw their development or stockpiling. Most major powers built up substantial chemical weapons reserves. In the 1930s, Italy employed chemical weapons against Ethiopia, and Japan used them against China. In World War II, chemical warfare did not occur, primarily because all the major belligerents possessed both chemical weapons and the defenses–such as gas masks, protective clothing, and detectors–that rendered them ineffectual. In addition, in a war characterized by lightning-fast military movement, strategists opposed the use of anything that would delay operations. Germany, however, did use poison gas to murder millions in its extermination camps.

Since World War II, chemical weapons have only been used in a handful of conflicts–the Yemeni conflict of 1966-67, the Iran-Iraq War of 1980-88–and always against forces that lacked gas masks or other simple defenses. In 1990, the United States and the Soviet Union signed an agreement to cut their chemical weapons arsenals by 80 percent in an effort to discourage smaller nations from stockpiling the weapons. In 1993, an international treaty was signed banning the production, stockpiling (after 2007), and use of chemical weapons. It took effect in 1997.