A subsidiary alliance is an alliance between a dominant nation and a nation that it dominates
British policy in India
The doctrine of subsidiary alliance was introduced by Marquess Wellesley, British governor-general of India from 1798 to 1805. In the beginning of his governorship Wellesley adopted a policy of non-intervention but later he adopted the policy of subsidiary alliance. By the late 18th century, powerful emperors had disappeared from the Indian subcontinent. and it was left with numerous weaker smaller states. Many rulers accepted this offer of protection by Lord Wellesley.
Its main principles were:
a) Any Indian Ruler accepting subsidiary Alliance with the British had to keep British forces within their territory and agreed to pay for their maintenance.
b) In lieu of the payments, some of the ruler's territory was ceded to the British.
c) The rulers not accepting it were forced to keep a British official at their court called the Resident.
Under this doctrine, Indian rulers under British protection suspended their native armies, instead maintaining British troops within their states. They surrendered control of their foreign affairs to the British. In return, the East India Company would protect them from the attacks of their rivals.
The Nizam of Hyderabad was the first to enter into this alliance. Tipu Sultan of Mysore refused to accept it but after the British victory in the Fourth Anglo-Mysore War, Mysore was forced to become a subsidiary ally. Later the Maratha ruler Baji Rao II also accepted subsidiary alliance in the Treaty of Bassein.