Cavalry
The French cavalry numbered 102,000 in May 1915 but had been reduced to 63,000 by October 1918. The German Army dismounted nearly all their cavalry in the West, maintaining only one mounted division on that front by January 1917.

Italy entered the war in 1915 with thirty regiments of line cavalry, lancers and light horse. While employed effectively against their Austro-Hungarian counterparts during the initial offensives across the Isonzo River, the Italian mounted forces ceased to have a significant role as the front shifted into mountainous terrain. By 1916 most cavalry machine-gun sections and two complete cavalry divisions had been dismounted and seconded to the infantry.
Some cavalry were retained as mounted troops in reserve behind the lines, in anticipation of a penetration of the opposing trenches that it seemed would never come. Tanks, introduced on the Western Front by the British in September 1916 during the Battle of the Somme, had the capacity to achieve such breakthroughs but did not have the reliable range to exploit them. In their first major use at the Battle of Cambrai (1917), the plan was for a cavalry division to follow behind the tanks, however they were not able to cross a canal because a tank had broken the only bridge. On a few other occasions, throughout the war, cavalry were readied in significant numbers for involvement in major offensives; such as in the Battle of Caporetto and the Battle of Moreuil Wood. However it was not until the German Army had been forced to retreat in the Hundred Days Offensive of 1918, that limited numbers of cavalry were again able to operate with any effectiveness in their intended role. There was a successful charge by the British 7th Dragoon Guards on the last day of the war.
In the wider spaces of the Eastern Front, a more fluid form of warfare continued and there was still a use for mounted troops. Some wide-ranging actions were fought, again mostly in the early months of the war. However, even here the value of cavalry was overrated and the maintenance of large mounted formations at the front by the Russian Army put a major strain on the railway system, to little strategic advantage. In February 1917, the Russian regular cavalry (exclusive of Cossacks) was reduced by nearly a third from its peak number of 200,000, as two squadrons of each regiment were dismounted and incorporated into additional infantry battalions. Their Austro-Hungarian opponents, plagued by a shortage of trained infantry, had been obliged to progressively convert most horse cavalry regiments to dismounted rifle units starting in late 1914.

Middle East: In the Middle East, during the Sinai and Palestine Campaign mounted forces (British, Indian, Ottoman, Australian, Arab and New Zealand) retained an important strategic role both as mounted infantry and cavalry.
In Egypt, the mounted infantry formations like the New Zealand Mounted Rifles Brigade and Australian Light Horse of ANZAC Mounted Division, operating as mounted infantry, drove German and Ottoman forces back from Romani to Magdhaba and Rafa and out of the Egyptian Sinai Peninsula in 1916.
After a stalemate on the Gaza–Beersheba line between March and October 1917, Beersheba was captured by the Australian Mounted Division's 4th Light Horse Brigade. Their mounted charge succeeded after a coordinated attack by the British Infantry and Yeomanry cavalry and the Australian and New Zealand Light Horse and Mounted Rifles brigades. A series of coordinated attacks by these Egyptian Expeditionary Force infantry and mounted troops were also successful at the Battle of Mughar Ridge, during which the British infantry divisions and the Desert Mounted Corps drove two Ottoman armies back to the Jaffa—Jerusalem line. The infantry with mainly dismounted cavalry and mounted infantry fought in the Judean Hills to eventually almost encircle Jerusalem which was occupied shortly after.
During a pause in operations necessitated by the German spring offensive in 1918 on the Western Front, joint infantry and mounted infantry attacks towards Amman and Es Salt resulted in retreats back to the Jordan Valley which continued to be occupied by mounted divisions during the summer of 1918.
The Australian Mounted Division was armed with swords and in September, after the successful breaching of the Ottoman line on the Mediterranean coast by the British Empire infantry XXI Corps was followed by cavalry attacks by the 4th Cavalry Division, 5th Cavalry Division and Australian Mounted Divisions which almost encircled two Ottoman armies in the Judean Hills forcing their retreat. Meanwhile, Chaytor's Force of infantry and mounted infantry in ANZAC Mounted Division held the Jordan Valley, covering the right flank to later advance eastwards to capture Es Salt and Amman and half of a third Ottoman army. A subsequent pursuit by the 4th Cavalry Division and the Australian Mounted Division followed by the 5th Cavalry Division to Damascus. Armoured cars and 5th Cavalry Division lancers were continuing the pursuit of Ottoman units north of Aleppo when the Armistice of Mudros was signed by the Ottoman Empire.

Post–World War I: A combination of military conservatism in almost all armies and post-war financial constraints prevented the lessons of 1914–1918 being acted on immediately. There was a general reduction in the number of cavalry regiments in the British, French, Italian and other Western armies but it was still argued with conviction (for example in the 1922 edition of the Encyclopædia Britannica) that mounted troops had a major role to play in future warfare. The 1920s saw an interim period during which cavalry remained as a proud and conspicuous element of all major armies, though much less so than prior to 1914.
Cavalry was extensively used in the Russian Civil War and the Soviet-Polish War. The last major cavalry battle was the Battle of Komarów in 1920, between Poland and the Russian Bolsheviks. Colonial warfare in Morocco, Syria, the Middle East and the North West Frontier of India provided some opportunities for mounted action against enemies lacking advanced weaponry.

The post-war German Army (Reichsheer) was permitted a large proportion of cavalry (18 regiments or 16.4% of total manpower) under the conditions of the Treaty of Versailles.
The British Army mechanised all cavalry regiments between 1929 and 1941, redefining their role from horse to armoured vehicles to form the Royal Armoured Corps together with the Royal Tank Regiment. The U.S. Cavalry abandoned its sabres in 1934 and commenced the conversion of its horsed regiments to mechanized cavalry, starting with the First Regiment of Cavalry in January 1933.
During the Turkish War of Independence, Turkish cavalry under General Fahrettin Altay was instrumental in the Kemalist victory over the invading Greek Army in 1922 during the Battle of Dumlupınar. The 5th Cavalry Division was able to slip behind the main Greek army, cutting off all communication and supply lines as well as retreat options. This forced the surrender of the remaining Greek forces and may have been the last time in history that cavalry played a definitive role in the outcome of a battle. 

During the 1930s, the French Army experimented with integrating mounted and mechanised cavalry units into larger formations. Dragoon regiments were converted to motorised infantry (trucks and motor cycles), and cuirassiers to armoured units; while light cavalry (chasseurs a' cheval, hussars and spahis) remained as mounted sabre squadrons. The theory was that mixed forces comprising these diverse units could utilise the strengths of each according to circumstances. In practice mounted troops proved unable to keep up with fast moving mechanised units over any distance.
The 39 cavalry regiments of the British Indian Army were reduced to 21 as the result of a series of amalgamations immediately following World War I. The new establishment remained unchanged until 1936 when three regiments were redesignated as permanent training units, each with six, still mounted, regiments linked to them. In 1938, the process of mechanization began with the conversion of a full cavalry brigade (two Indian regiments and one British) to armoured car and tank units. By the end of 1940, all of the Indian cavalry had been mechanized, initially and in the majority of cases, to motorized infantry transported in 15cwt trucks. The last horsed regiment of the British Indian Army (other than the Viceroy's Bodyguard and some Indian States Forces regiments) was the 19th King George's Own Lancers which had its final mounted parade at Rawalpindi on 28 October 1939. This unit still exists in the Pakistan Army as an armored regiment.

World War II: While most armies still maintained cavalry units at the outbreak of World War II in 1939, significant mounted action was largely restricted to the Polish, Balkan, and Soviet campaigns. Rather than charge their mounts into battle, cavalry units were either used as mounted infantry (using horses to move into position and then dismounting for combat) or as reconnaissance units (especially in areas not suited to tracked or wheeled vehicles).

Polish: A popular myth is that Polish cavalry armed with lances charged German tanks during the September 1939 campaign. This arose from misreporting of a single clash on 1 September near Krojanty, when two squadrons of the Polish 18th Lancers armed with sabres scattered German infantry before being caught in the open by German armoured cars.
Two examples illustrate how the myth developed.