Two German soldiers and their mule wearing gas masks, 1916

Many animals were used during World War I. Horses, mules, dogs and pigeons were exposed to poisonous gases, so special protection was necessary for them. Horses were equipped with gas masks on their muzzles and were protected from inhalation of toxic gases such as phosgene.

The horse's eyes were not affected by lachrymatory agents, so their masks consisted only of specially made nasal bags, but unfortunately, the eyes of these animals were vulnerable to the effects of chlorine and vesicatory gases.

Of the millions of British horses sent overseas to help with the war effort, only 62,000 returned home. It is the forgotten tragedy of the Great War - a conflict that, like humans, put many animals in the line of fire. For many years, little was known about the unimaginable suffering of animals being carried across the Channel to the Western Front.

In 1914, both sides had large cavalry. Horse and camel-mounted troops were used in desert campaigns throughout the war, but on the Western Front, new weapons such as machine guns made cavalry charges more difficult.

However, the animals remained an important part of the war effort. Horses, donkeys, mules and camels carried food, water, ammunition, and medical supplies to the people of the front, and dogs and pigeons carried messages. Canaries were used to detect the poisonous gas, and cats and dogs were trained to hunt rats in the trenches.

The Hague Declaration of 1899 and the Hague Convention of 1907 forbade the use of "poisonous or poisonous weapons" in war, yet by the end of World War I more than 124,000 tons of gas had been produced. The French were the first to use chemical weapons. During the Great War, using tear gases, ethyl bromoacetate and chloroacetone.

One of Germany's earliest uses of chemical weapons was on October 27, 1914, when shells containing irritant dinisidine chlorosulfonate were fired at British troops near Neuve-Chapelle, France.

Germany used another irritant, xylyl bromide, to fire artillery shells into what is now Poland, at the Russians near the town of Bolimo in January 1915.

The first full-scale deployment of lethal chemical warfare agents during World War I occurred at the Second Battle of Ypres on April 22, 1915, when the Germans attacked French, Canadian and Algerian troops with chlorine gas.

The deaths were mild, although the casualties were relatively heavy. A total of 50,965 tons of pulmonary, lachrymatory and vesicant agents were deployed by both sides of the conflict, including chlorine, phosgene and mustard gas.

Official figures declare 1,176,500 nonfatal casualties and 85,000 deaths directly attributable to chemical warfare agents during the war.