What do you think makes a "nation"? Race, language or religion? I think a nation is made by people who share a culture. And the culture of India is Hindu and has been Hindu for millenia.
The concept of state is different. The state is a political organisation working with the broad assent of the people constituting a nation or nations. While states change, a nation remains the same.
What you say is true - the Hindu nation was earlier organised into different kingdoms, and that state was superceded by the Indian state 60 years ago. And Bharat, as a cultural unit, was not formed 200-300 years ago, but much before that.
Of course North, South, East, West are all different, and no one is saying they are not. But that does not mean that a person from one part should be thought of and treated like a foreigner in another part - not only because we are all citizens of the Indian state, but because the genius of Hindu civilization has allowed us all to retain our own sub-cultures, yet feel integrated into a larger nation.
Lastly, my post above was mainly in response to Mr. Tamil natan who said that Krishna is fine, but Ram is a foreigner. I asked him if the nonsense that EVR peddled made the nonsense true. What do you think?