The Imperial Service Troops were forces raised by the princely states of the British Indian Empire. These troops were available for service alongside the Indian Army when such service was requested by the British government. At the beginning of the 20th century, their total numbers were about 18,000 men.
The troops were routinely inspected by British army officers and generally had the same equipment as soldiers in the Indian Army. Although their numbers were relatively small, the Imperial Service Troops were employed in China and British Somaliland in the first decade of the 20th century, and later saw action in the First World War and Second World War.
In March 1885, after a Russian force defeated the Afghan army at Panjdeh, a village on the ill-defined Russian-Afghan frontier, the British were alarmed and immediately dispatched units from the Army of India to reinforce the North-West Frontier defenses. The "Panjdeh incident," however, also resulted in generous donations of money for a potential war effort from many Indian rulers, most notably the Nizam of Hyderabad. Although the incident was later resolved by a compromise, the timely support of the Indian princes prompted the British to create a reserve force of approximately 20,000 soldiers recruited from the armies of the Native States, but trained and equipped by the Indian Army. The cost of the training and new equipment was borne by the Native States.
The armies of the Princely States were bound by many restrictions that were imposed by subsidiary alliances. They existed mainly for ceremonial use and for internal policing. According to the Imperial Gazetteer of India vol. IV 1907, p. 85,
"Since a chief can neither attack his neighbour nor fall out with a foreign nation, it follows that he needs no military establishment which is not required either for police purposes or personal display, or for cooperation with the Imperial Government. The treaty made with Gwalior in 1844, and the instrument of transfer given to Mysore in 1881, alike base the restriction of the forces of the State upon the broad ground of protection. The former explained in detail that unnecessary armies were embarrassing to the State itself and the cause of disquietude to others: a few months later a striking proof of this was afforded by the army of the Sikh kingdom of Lahore. The British Government has undertaken to protect the dominions of the Native princes from invasion and even from rebellion within: its army is organized for the defence not merely of British India, but of all the possessions under the suzerainty of the King-Emperor."