The Deccan Horse or 9 Horse is one of the oldest and most decorated armoured regiments of the Indian Army. The Royal Deccan Horse (9th Horse), which was a regular cavalry regiment of the British Indian Army was formed from the amalgamation of two regiments after World War I. They saw service from the Mutiny of 1857 up to and including World War II.

The 9th Royal Deccan Horse can trace its formation to 1790 when it was called Asif Sah's Irregular Cavalry. Two regiments were raised for service under the Nizam of Hyderabad in Berar, who was allied with the British East India Company.

During the following years, the titles of these two Regiments went through many changes. They were known by the following titles over the years:

In 1903, during Kitchener's reform of the Indian Army, the 'Hyderabad Contingent' was incorporated into the regular British Indian Cavalry as the two regiments, the 20th Deccan Horse and 29th Lancers (Deccan Horse). In 1921, 20th Deccan Horse was given the title "Royal" for its distinguished service during World War I. In 1921, the two regiments were merged under the name "The 9th Royal Deccan Horse". In 1927, the designation was changed to "The Royal Deccan Horse (9th Horse)". The regimental class composition at the time of the Hyderabad Contingent was a squadron each of Sikhs, Deccani Mussalmans and Jats. In 1927, the composition was of Punjabi Muslims, Sikhs and Jats. On India becoming a Republic in 1950, the regiment's name was finally changed to "The Deccan Horse (9th Horse)".

The Deccan Horse which remained a local corps in the Deccan, was frequently called for service during the 18th and 19th centuries. This included operations in the frontier to check the incursions of the Pindaris (1811), in Aurangabad against the Naiks in Central Berar (1817), the Third Anglo-Maratha War of 1817–18, sieges of Nowah and Umerkher in January 1819, in operations in the Raichur Doab (1819), against the Bhils in Kingaon, attack on the fort at Dandoti (1828), against Bhils in Lonar (1829), attack on the fort of Kaptak (1830), seizure of the fort in Nandgaon (1832), capture of the fort at Nanand (1833), against the Bhils at Jafferabad and Dhar, north of Jalna, against the Arabs near Afzalpur (1841), against the Rohillas at Warur and Bairugarh (1842), against the Rohillas near Nander and at the capture of the fort of Saorgaon (1842), against Bhils (1845–46), attack on the fort at Kandat (1847), in the quelling of a Rohilla rebellion and in the capture of the fort of Rai Mhow (1848), siege of Dharur (1851) and the siege of Sailur (1854).

Following the formation of the Hyderabad Contingent, the unit saw action in many locations around and after the Indian Rebellion of 1857 (Dhar, Jhansi, Piplia, Betwa River, Rawal, Banda, Mandesur, Kunch, Ratgarh, Kalpi, Madanpur, Bilawa, Chanderi, Gwalior and Garakota) winning a Victoria Cross (VC) in 1859 and was also awarded the battle honour, 'Central India' for its efforts in suppression of the mutiny in 1857–58.

Officers and troops from the regiment took part in the Second Afghan War, Tirah campaign, Kurram Valley Field Force (1879–1880), the Burma War (1886–1888), in Central Africa (1891–1892), in the North-West Frontier expedition (1897–1898), in China during the Boxer Rebellion (1900) and in the South African War (1900–1901).

The 20th Deccan Horse was sent to France for service on the Western Front. The regiment was stationed in Bolarum before the war. It sailed from Bombay on board the SS Ellora and HMT Egra on 15 September 1914 to Marseille. It was part of the 9th (Secunderabad) Cavalry Brigade of the 2nd Indian Cavalry Division. The regiment remained in the vicinity or Marseille from 12 to 23 October 1914, following which it moved to Orléans. It moved to Vieille-Chapelle in November and saw fighting around Festubert in November and December 1914. It took part in the attack on Givenchy and took heavy casualties. The regiment remained behind the lines during most of 1915.