India was among the hardest of major civilizations to conquer. Over the past 3000+ years very few outside powers were able to break past the Indus river forming India’s northwest boundary. Let’s look at the maps of some of the greatest conquests. You can notice that all of them stop near the present Indian border.
Achaemenid Empire (Persia)
Not just the conquest from the western side, even from the north it was the same story.
Genghis Khan & Mongols
Question is how could all these great conquerors – Alexander, Darius, Genghis Khan had to stop at the present Indian border. Part of it is out geography and part of it is due to the strong domestic empires. Contrary to popular myth, Indian empires – especially those near the border were tough fighters. Even a small local ruler like Puru could fight Alexander’s marauding troops at an equal footing.
One mistake people often make is in the definition of invasion. An invasion means a part of your territory becomes a part of someone else’s territory. For instance, Britain invaded India as India became a part of somebody else’s political unit. Romans invaded Britain, because the territory began to be ruled from Rome. In most attacks in case of India, it was merely attacks/raids on a part of the country that was quickly reversed. This was the case in case of Ghazni’s or Timur’s attacks – not too much different from the terror attacks of the modern day experienced in Mumbai and elsewhere.
The ones who could break past the defenses, found it hard to capture any more than say 10% of the territory. Most often it would be in the northwest corner of India.
The few who could really get past through had a tough time staying put. Thus, historically no major part of subcontinent really became a part of another empire.
For instance, Babur won a key battle for India’s north, but in a few years his empire went to a naught. It was his grandson who actually built an empire and he was as much an Indian like any other. This is the next main story – the few who really manage to both break and stay put become Indian in every sense.
Akbar was born in India, ruled in India, died in India, thought like an Indian and I don’t see why his empire was not Indian. British Raj was the only real exception and Britain was capturing all over the world. Innovations in textile production and railways also helped it. Even there, the empire lasted only little more than 150 years and India emerged without much changes to the society or demographics. Had Britain stayed a few more years in India, it risked getting Indianized itself [already you can notice curry and Indian origin words in British culture]. By moving out quick, Britain escaped Indianization while all of its other “conquerors” did not.
This is why India among the large civilizations has a direct link to its iron age civilization – in language, culture, religion, name etc in a fairly unbroken way. In case of every other civilization, the names [as seen from the outside], religions all vanished with time that the religion of the past are restricted to the museums. There is not a single major civilization that has not had sizable amount of external attacks & invasions.