I am aware of the origin of the words India and Hindu which I find bemusing.
> So people visiting/invading the eastern parts of the Indu/Sindhu river were considering the people there as one civilisation if not one country.
I conjecture it is natural for foreigners at that time to refer to the land (and hence the people residing there) east of Sindhu river as India/Indians. For them it was just an alien/different region. For example, to refer Asoka as Indian ruler as in "king who belongs to India" and not "Ruler of India".
I'm curious to know if the contemporary people living in different parts of Indian land felt as if they belonged to India as a whole. Though possible, it is highly unlikely IMO. I highly doubt if a farmer in a village in Kerala would consider a village in Punjab as belonging to his nation.
> But there were attempts made before, as far back as Chanakya, to unify the people under one rule.
I've read/seen this in popular stories (Chanakya serial from 1990s is decent) but I don't know how authentic they are. I'd love to read more on this topic, preferably by historians. My naive search was fruitless. One thing to note is that Indian historical records from that period are extremely rare making it all the more difficult. It's a pity that Indians of that didn't write as much as Romans did.