Following is an excerpt from Machiavelli's "The Prince" which is considered one of his best pieces and is widely used in many places to determine foreign policy. It might very well be useful to determine our foreign policy too. I advise reading all of his classics. Most of them are almost free.
(Warning: If you're thinking that the Indian Subcontinent consists of only the Indian State, you can leave the post now. If you're thinking it is still a bunch of states that deserve to be united in favor of the most powerful state, you are my kind.)
From Ch. 3 Mixed Principalities:
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Moreover, a prince who occupies a province which, as previously described, differs from his own, must become the leader and defender of the less powerful neighboring states and seek to weaken the more powerful among them. He must also be on guard lest by any chance a foreigner equal to him in power should enter them. Such an event always comes about through the help of discontented inhabitants who willingly admit a foreign power either through excessive ambition or through fear, as was the case with the Etolians, who admitted the Romans into Greece. So, it was also with every province that the Romans entered, they were brought in by the inhabitants themselves. It is in the nature of things that as soon as a powerful foreigner enters a province, all the weaker powers in it will become his allies through envy of those who have been ruling over them. This is so true that, with respect to minor powers, the invader need do nothing at all to win them, for they will all willingly merge in the state which he has acquired. He has but to see to it that they do not gain too much strength and authority. With his own forces and their support, he can very early reduce the stronger powers and then become arbiter of the entire province. Any ruler who does not succeed in doing this will soon loose what he has won, or so long as he does manage to hold it, will have a host of difficulties and annoyances. The Romans very carefully observed this policy in the provinces they conquered. They sent out colonies; they protected the lesser powers without increasing their strength; they reduced those who were strong, and they did not permit powerful foreigners to gain a footing. Their conduct in Greece will suffice as an example: there the Romans protected the Achaeans and the Etolians, reduced the kingdom of the Macedonians, and expelled Antiochus. Nor did they ever reward the Achaeans and the Etolians by allowing them to enlarge their states, or allow Philip to persuade them to become his friends until they had weakened him. Nor did the power of Antiochus ever induce them to permit his keeping any part of Greece. In this instance the Romans did everything that wise princes should do who must have regard not only for existing disorders but for future ones as well, avoiding them with all possible diligence.
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