Here , a definition of “India” is essential. And India needs to be seen in terms of a civilizational nation. Meaning, that the civilisation from Kashmir to Kanyakumari and Balochistan to Burma is the same. There are other civilisational countries – primary among them China and Japan.
Politically, Assam has been united with “India” perhaps only since the advent of the British and the treaty of Yandaboo of 1826. But by that definition political unification of even the rest of India has happened only during the reigns of Chandragupta , Ashok , Allaudin Khilji and Aurangzeb.
Does that negate the cultural oneness of this land ? Once we understand that the European model is not the only one to look at history with, we can see the difference. They have always identified themselves with narrow definitions of French, German, English, Danish etc — even today there is no overarching European identity as such. Not so with India.
So coming to Assam … the region is most definitely mentioned in Mahabharat. Bhagadutta, who fought on side of Kauravas is from Assam. Other characters like Rukmini, Chitrangada, Ghatotkach etc are also associated with the NE. The Kacharis who ruled Dimapur even today show tall carved stones as “dice Kauravas and Pandavas played with” . Now I am not going to ascertain whether those were the dice, but the point is that the Mahabharat is deeply ingrained in Assam.
We have the Shakti peeth at Kamakhya. The Shaktipeeths, like the Jyotirling are a good indication of the notion of “rashtra” as seen by ancient people. The Shakti peeths are definite. Hingalaj Mata in Balochistan represents the head and only that represents the head. 50 other peeths don’t get up and stake claim to it.
Similarly, the Yoni is represented by Kamakhya. Thus, we can see that the second link of NE to mainland India is the Shakti peeth.
Then we have various dynasties such the Salstambh etc , all Hindu, ruling over Assam.
In the 15th century we have Chilarai or Sukhladhwaj, who united many across Assam, gave refuge to Shrimant Shankaradev , rebuilt the Kamakhya temple destroyed by Hussain Shah etc. The Yogini Tantra was also composed in Assam.
From 1228 onwards we have the Ahoms, who ruled over Assam , patronised the temples and fought off the Mughals. Slowly but surely, they integrated themselves more and more with the Hindu majority they were ruling. They represented a culture diametrically opposite to the Turks, Persians and Uzbeks ruling most of India at the time. Although migrants, the Tai Ahom adopted and enriched the local culture. They did not try to trample and eliminate it. Thus, their ethos were entirely “Indian” .
In the 18th century we have Rudra Singha, who patronised Hindu religion and Sanskrit. Built many temples , tanks. Joisagar tank is among the biggest in India even today. He even sent scholars to Varanasi for studying and wanted to extend his empire at least to the banks of the Ganges.
Even in 1857, when the whole of Hindustan had been engulfed by the War of Independence, Maniram Dewan was inspired by the events to carry out an identical uprising against the British in Assam too.
Thus, we see that politically it might be part of India only since 1826 , but culturally it has been united with India since centuries and millenia past. And this view should be known and become popular .