The use of horses in World War I (1914–1918) marked a transitional period in the evolution of armed conflict. Cavalry units were initially considered essential offensive elements of a military force, but over the course of the war, the vulnerability of horses to modern machine gun, mortar, and artillery fire reduced their utility on the battlefield. This paralleled the development of tanks, which ultimately replaced cavalry in shock tactics. While the perceived value of the horse in war changed dramatically, horses still played a significant role throughout the war.

All of the major combatants in World War I began the conflict with cavalry forces. Imperial Germany stopped using them on the Western Front soon after the war began, but continued with limited use on the Eastern Front, well into the war. The Ottoman Empire used cavalry extensively during the war. On the Allied side, the United Kingdom used mounted infantry and cavalry charges throughout the war, but the United States used cavalry only briefly. Although not particularly successful on the Western Front, Allied cavalry had some success in the Middle Eastern theatre due to the open nature of the front, allowing a more traditional war of movement, in addition to the lower concentration of artillery and machine guns. Russia used cavalry forces on the Eastern Front but with limited success.

The military used horses mainly for logistical support; they were better than mechanized vehicles at traveling through deep mud and over rough terrain. Horses were used for reconnaissance and for carrying messengers as well as for pulling artillery, ambulances, and supply wagons. The presence of horses often increased morale among the soldiers at the front, but the animals contributed to disease and poor sanitation in camps, caused by their manure and carcasses. The value of horses and the increasing difficulty of replacing them were such that by 1917, some troops were told that the loss of a horse was of greater tactical concern than the loss of a human soldier. Ultimately, the blockade of Germany prevented the Central Powers from importing horses to replace those lost, which contributed to Germany's defeat. By the end of the war, even the well-supplied US Army was short of horses.

Conditions were severe for horses at the front; they were killed by rifle and artillery fire, suffered from skin disorders among other diseases, and were injured by poison gas. Hundreds of thousands of horses died, and many more were treated at veterinary hospitals and sent back to the front. Procuring fodder was a major issue, and Germany lost many horses to starvation. Several memorials have been erected to commemorate the horses that died. Artists, including Alfred Munnings, extensively documented the work of horses in the war, and horses were featured in war poetry. Novels, plays and documentaries have also featured the horses of World War I.

Cavalry

Many British tacticians outside of the cavalry units realized before the war that advances in technology meant that the era of mounted warfare was coming to an end. However, many senior cavalry officers disagreed, and despite limited usefulness, maintained cavalry regiments at the ready throughout the war. Scarce wartime resources were used to train and maintain cavalry regiments that were rarely used. The continued tactical use of the cavalry charge resulted in the loss of many troops and horses in fruitless attacks against machine guns.Ellis, Cavalry, pp. 174–76 Early in the war, cavalry skirmishes occurred on several fronts, and horse-mounted troops were widely used for reconnaissance.Willmott, First World War, p. 46 Britain's cavalry were trained to fight both on foot and mounted, but most other European cavalry solely relied on the shock tactic of mounted charges. There were isolated instances of successful shock combat on the Western Front, where cavalry divisions also provided important mobile firepower. Beginning before the war, cavalry was deployed alongside armoured cars and aircraft, this was later extended to working in co-operation with tanks, notably at the Battle of Cambrai, where cavalry was expected to exploit breakthroughs in the lines that the slower tanks could not. This plan never came to fruition due to missed opportunities by command. At Cambrai, troops from Great Britain, Canada, India and Germany participated in mounted actions.Hammond, Cambrai 1917, pp. 69, 450–51 Cavalry was still deployed late in the war, with Allied cavalry troops harassing retreating German forces in 1918 during the Hundred Days Offensive, when horses and tanks continued to be used in the same battles. "Cavalry was literally indispensable" on the Eastern front and in the Middle East.

The Cavalry Manual, written in 1912, was unchanged throughout the war. Douglas Haig believed that the cavalry arm could also be effective strategically. This was proven to be wrong and the blame is laid with the higher echelons who were unable to establish a Command, Control and Communications structure which could effectively involve the use of cavalry divisions or corps.Kenyan Horsemen In No Man's Land, British Cavalry & Trench Warfare 1914-1918 p14 Cavalry was also used with good effect in Palestine, at the Third Battle of Gaza and Battle of Megiddo.Carver, Britain's Army in the 20th Century, p. 123 British cavalry officers, far more than their continental European counterparts, persisted in using and maintaining cavalry, believing that mounted troops would be useful for exploiting infantry breakthroughs, and under the right circumstances would be able to face machine guns.Meyer, A World Undone, p. 264

British Empire

United Kingdom

Britain had increased its cavalry reserves after seeing the effectiveness of mounted Boers during the Second Boer War (1899–1902).Dent, Cleveland Bay Horses, pp. 61–64 Horse-mounted units were used from the earliest days of World War I: on August 22, 1914, the first British shot of the war in France was fired by a cavalryman, Corporal Edward Thomas of the 4th Royal Irish Dragoon Guards, near Casteau, during a patrol in the buildup to the Battle of Mons. Within 19 days of Britain beginning mobilization for war, on August 24, 1914, the 9th Lancers, a cavalry regiment led by David Campbell, engaged German troops with a squadron of 4th Dragoon Guards against German infantry and guns. Campbell obeyed his orders to charge, although he believed the more prudent course of action would have been to fight dismounted. The charge resulted in a British loss of 250 men and 300 horses. On September 7, Campbell's troops charged again, this time towards the German 1st Guard Dragoons, another lancer cavalry regiment.

The Cavalry Training Manual of 1912 continued to be used by junior commanders to great effect (significantly Douglas Haig was a contributor) and was not changed significantly during the war years. What did change however, to the advantage of small cavalry formations, was how the German army changed tactics (to deep defence) allowing the cavalry more freedom on the battlefield. The last British fatality from enemy action before the armistice went into effect was a cavalryman, George Edwin Ellison, from C Troop 5th Royal Irish Lancers. Ellison was shot by a sniper as the regiment moved into Mons on November 11, 1918.

Horses proved indispensable to the British war effort in Palestine, particularly under Field Marshal Edmund Allenby, for whom cavalry made up a large percentage of his forces. Most of his mounted troops were not British regular cavalry, but the Desert Mounted Corps, consisting of a combination of Australian, New Zealand, Indian units and English Yeomanry regiments from the Territorial Force, largely equipped as mounted infantry rather than cavalry. By mid-1918, Ottoman intelligence estimated that Allenby commanded around 11,000 cavalry.Erickson, Ordered to Die, pp. 195–97 Allenby's forces routed the Ottoman armies in a running series of battles that included the extensive use of cavalry by both sides. Some cavalry tacticians view this action as a vindication of cavalry's usefulness, but others point out that the Ottoman were outnumbered two to one by late 1918, and were not first-class troops.Ellis, Cavalry, pp. 176–77 Horses were also ridden by the British officers of the Egyptian Camel Transport Corps in Egypt and the Levant during the Sinai and Palestine Campaigns.McPherson, et al., The man who loved Egypt, pp. 184–86

India Indian cavalry participated in actions on both the Western and Palestinian fronts throughout the war. Members of the 1st and 2nd Indian Cavalry Divisions were active on the Western Front, including in the German retreat to the Hindenburg Line and at the Battle of Cambrai. During the battle of the Somme, the 20th Deccan Horse made a successful, mounted charge, assaulting a German position on Bazentin Ridge. The charge overran the German position. A charge by the 5th (Mhow) Cavalry Brigade of the 1st Division ended successfully at the Battle of Cambrai despite being against a position fortified by barbed wire and machine guns. Such successful endings were unusual occurrences during the war.Hammond, Cambrai 1917, pp. 396–402 Several Indian cavalry divisions joined Allenby's troops in the spring of 1918 after being transferred from the Western Front.

Canada

When the war began, Lord Strathcona's Horse, a Canadian cavalry regiment, was mobilized and sent to England for training. The regiment served as infantry in French trenches during 1915, and were not returned to their mounted status until February 16, 1916. In the defense of the Somme front in March 1917, mounted troops saw action, and Lieutenant Frederick Harvey was awarded the Victoria Cross for his actions. Canadian cavalry generally had the same difficulties as other nations in breaking trench warfare deadlocks and were of little use on the front lines. However, in the spring of 1918, Canadian cavalry was essential in halting the last major German offensive of the war. On March 30, 1918, Canadian cavalry charged German positions in the Battle of Moreuil Wood, defeating a superior German force supported by machine gun fire. The charge was made by Lord Strathcona's Horse, led by Gordon Flowerdew, later posthumously awarded the Victoria Cross for his actions during the charge. Although the German forces surrendered, three-quarters of the 100 cavalry participating in the attack were killed or wounded in the attack against 300 German soldiers.

Australia and New Zealand