Cavalry
11th Prince Albert Victor's Own Cavalry (Frontier Force) (now 11th Cavalry (Frontier Force), Pakistan)
Several of these formations are still active, though they now are armoured formations, for example the Guides Cavalry of Pakistan.

The French Army maintained substantial cavalry forces in Algeria and Morocco from 1830 until the end of World War II. Much of the Mediterranean coastal terrain was suitable for mounted action and there was a long established culture of horsemanship amongst the Arab and Berber inhabitants. The French forces included Spahis, Chasseurs d' Afrique, Foreign Legion cavalry and mounted Goumiers. Both Spain and Italy raised cavalry regiments from amongst the indigenous horsemen of their North African territories (see regulares, Italian Spahis and savari respectively).
Imperial Germany employed mounted formations in South West Africa as part of the Schutztruppen (colonial army) garrisoning the territory.

United States: In the early American Civil War the regular United States Army mounted rifle, dragoon, and two existing cavalry regiments were reorganized and renamed cavalry regiments, of which there were six. Over a hundred other federal and state cavalry regiments were organized, but the infantry played a much larger role in many battles due to its larger numbers, lower cost per rifle fielded, and much easier recruitment. However, cavalry saw a role as part of screening forces and in foraging and scouting. The later phases of the war saw the Federal army developing a truly effective cavalry force fighting as scouts, raiders, and, with repeating rifles, as mounted infantry. The distinguished 1st Virginia Cavalry ranks as one of the most effectual and successful cavalry units on the Confederate side. Noted cavalry commanders included Confederate general J.E.B. Stuart, Nathan Bedford Forrest, and John Singleton Mosby (a.k.a. "The Grey Ghost") and on the Union side, Philip Sheridan and George Armstrong Custer.
Post Civil War, as the volunteer armies disbanded, the regular army cavalry regiments increased in number from six to ten, among them Custer's U.S. 7th Cavalry Regiment of Little Bighorn fame, and the African-American U.S. 9th Cavalry Regiment and U.S. 10th Cavalry Regiment. The black units, along with others (both cavalry and infantry), collectively became known as the Buffalo Soldiers. According to Robert M. Utley: 

the frontier army was a conventional military force trying to control, by conventional military methods, a people that did not behave like conventional enemies and, indeed, quite often were not enemies at all. This is the most difficult of all military assignments, whether in Africa, Asia, or the American West.
These regiments, which rarely took the field as complete organizations, served throughout the American Indian Wars through the close of the frontier in the 1890s. Volunteer cavalry regiments like the Rough Riders consisted of horsemen such as cowboys, ranchers and other outdoorsmen, that served as a cavalry in the United States Military.

Developments 1900–1914: At the beginning of the 20th century, all armies still maintained substantial cavalry forces, although there was contention over whether their role should revert to that of mounted infantry (the historic dragoon function). With motorised vehicles and aircraft still under development, horse mounted troops remained the only fully mobile forces available for manoeuvre warfare until 1914.

England: Following the experience of the South African War of 1899–1902 (where mounted Boer citizen commandos fighting on foot from cover proved more effective than regular cavalry), the British Army withdrew lances for all but ceremonial purposes and placed a new emphasis on training for dismounted action in 1903. Lances were however readopted for active service in 1912.

Russia: In 1882, the Imperial Russian Army converted all its line hussar and lancer regiments to dragoons, with an emphasis on mounted infantry training. In 1910 these regiments reverted to their historic roles, designations and uniforms.

Germany: By 1909, official regulations dictating the role of the Imperial German cavalry had been revised to indicate an increasing realization of the realities of modern warfare. The massive cavalry charge in three waves which had previously marked the end of annual maneuvers was discontinued and a new emphasis was placed in training on scouting, raiding and pursuit; rather than main battle involvement. The perceived importance of cavalry was however still evident, with thirteen new regiments of mounted rifles (Jäger zu Pferde) being raised shortly before the outbreak of war in 1914.

France: In spite of significant experience in mounted warfare in Morocco during 1908–14, the French cavalry remained a highly conservative institution. The traditional tactical distinctions between heavy, medium, and light cavalry branches were retained. French cuirassiers wore breastplates and plumed helmets unchanged from the Napoleonic period, during the early months of World War I. Dragoons were similarly equipped, though they did not wear cuirasses and did carry lances. Light cavalry were described as being "a blaze of colour". French cavalry of all branches were well mounted and were trained to change position and charge at full gallop. One weakness in training was that French cavalrymen seldom dismounted on the march and their horses suffered heavily from raw backs in August 1914.

First World War: 

Opening stages: 

Europe 1914: In August 1914, all combatant armies still retained substantial numbers of cavalry and the mobile nature of the opening battles on both Eastern and Western Fronts provided a number of instances of traditional cavalry actions, though on a smaller and more scattered scale than those of previous wars. The 110 regiments of Imperial German cavalry, while as colourful and traditional as any in peacetime appearance, had adopted a practice of falling back on infantry support when any substantial opposition was encountered. These cautious tactics aroused derision amongst their more conservative French and Russian opponents but proved appropriate to the new nature of warfare. A single attempt by the German army, on 12 August 1914, to use six regiments of massed cavalry to cut off the Belgian field army from Antwerp floundered when they were driven back in disorder by rifle fire. The two German cavalry brigades involved lost 492 men and 843 horses in repeated charges against dismounted Belgian lancers and infantry. One of the last recorded charges by French cavalry took place on the night of 9/10 September 1914 when a squadron of the 16th Dragoons overran a German airfield at Soissons, while suffering heavy losses. Once the front lines stabilised on the Western Front with the start of Trench Warfare, a combination of barbed wire, uneven muddy terrain, machine guns and rapid fire rifles proved deadly to horse mounted troops and by early 1915 most cavalry units were no longer seeing front line action.
On the Eastern Front, a more fluid form of warfare arose from flat open terrain favorable to mounted warfare. On the outbreak of war in 1914 the bulk of the Russian cavalry was deployed at full strength in frontier garrisons and, during the period that the main armies were mobilizing, scouting and raiding into East Prussia and Austrian Galicia was undertaken by mounted troops trained to fight with sabre and lance in the traditional style. On 21 August 1914 the 4th Austro-Hungarian Kavalleriedivison fought a major mounted engagement at Jaroslavic with the Russian 10th Cavalry Division, in what was arguably the final historic battle to involve thousands of horsemen on both sides. While this was the last massed cavalry encounter on the Eastern Front, the absence of good roads limited the use of mechanized transport and even the technologically advanced Imperial German Army continued to deploy up to twenty-four horse-mounted divisions in the East, as late as 1917.

Europe 1915–1918: For the remainder of the War on the Western Front, cavalry had virtually no role to play. The British and French armies dismounted many of their cavalry regiments and used them in infantry and other roles: the Life Guards for example spent the last months of the War as a machine gun corps; and the Australian Light Horse served as light infantry during the Gallipoli campaign. In September 1914 cavalry comprised 9.28% of the total manpower of the British Expeditionary Force in France—by July 1918 this proportion had fallen to 1.65%. As early as the first winter of the war most French cavalry regiments had dismounted a squadron each, for service in the trenches. 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.