Cough, French Indochina, cough?
Thande,
You should really get that cough checked out, and the knee jerk that accompanies that cough too.
In Vietnam, the US was fighting against communism and not fighting against the dissolution of the French colonial empire. The thinking behind it was was about "preserving a free country" and not about "preserving a French colony".
Now, we all know and have known for some time that such thinking was wrong. It was the wrong war in the wrong place at the wrong time. However, the ideas behind the war didn't involve putting a French colonial government back in power, so the war wasn't in support of imperialism.
Getting back to the OP's question now...
India is unified to the extent is now because the British Raj unified it. It remained unified to the extent it did after "independence" because the Indian elites wanted it to remain unified. The masses in 1949 had nothing to do with this because they had as much "political consciousness" as a cherrystone clam. As was done in in nearly every former European colony, local elites merely stepped into the shoes of the departing European rulers and nothing much actually changed.
Immediately prior to and after independence, the new rulers of India made what I consider to be a series of very pragmatic choices. First, they hived off two majority Muslim enclaves (and the real problems they represented) into the two parts of Pakistan. Second, they crushed - just like any other colonial power would have crushed - the other independence movements that might have further fractured the unified state their European predecessors left them(1). Remember, Hyderabad was only one of the potential "nations" that were crushed. In these two acts, India's new rulers chose to keep as much of the Raj-unified India as they felt they could control and India's history since independence suggests they chose wisely.
Of course, India has changed and grown since those choices were made in 1940s and 1950s. We need to remember what India and her people were like during that period to understand why events occurred the way they did.
Bill
1 - Indonesia is another example of this. While the Dutch suggested that a post-colonial federation be created for the islands, but the local elites kept the same centralized system in place replacing Dutch officials with Javanese and changing essentially nothing. The "nation" that resulted was a Javanese empire as surely as the "colony" it replaced was part of the Dutch empire.