Lord Wellesley had two aims: Firstly, the British Empire should be well-defended and should have no danger from any external power. Secondly, the Empire should expand as widely as possible so that, in course of time, the whole of India might become a British possession.
To achieve both objectives, he wanted to bring the Indian princes under the supremacy of the British. Once that was done, there should be no fear of foreign invasion, because, the princes could be no more under any foreign influence. Side by side, those who would refuse to accept the British supremacy should be conquered. Such was the policy of Lord Wellesley. In order to work out that policy, he adopted a political principle, which came to be known as the Subsidiary Alliance. Wellesley invited the Indian rulers to accept that Alliance.
The following were the terms of the Alliance. Firstly, those Indian rulers who would accept the Subsidiary Alliance should agree not to fight with any other power without the permission of the British. They should not also keep any contact with any outside power. Secondly, in each big state which would enter into the Subsidiary Alliance, there should be an army to maintain internal peace and order. That army should remain under the control of the English generals. To meet the cost of that army, a portion of that state should be surrendered to the British Government. Thirdly, in each small state that would enter the Alliance, there should also be an army like that. To meet the cost of that army, a small state should not surrender any part of its territory. It should only pay an annual amount to the British Government. Fourthly, in return of such conditions, the British Government should protect all states, small or big, against enemy invasions.
That was the essence of the Subsidiary Alliance. The real intention was to extinguish the independence of the Indian rulers and to give them security under the British military might. Wellesley threw the offer to the kings of India to accept the Alliance and remain safe.