The location is such that if Dhaka wanted, they could have annexed the region militarily too. But they were clever enough not to waste blood and bullets and instead took the sublime route of sending its population over the poorly guarded Indian border.
The silent invasion started way back in the British era, when the area was clubbed with Bangladesh as a single political Division. It happened due to lack of Nationalism in the part of the so called "Assamese" populace which is even missing today. The Muslims had declared their will to club Assam into a Greater Bangladesh way back publicly and as a part of that plan came waves of Bangladeshis into the Brahmaputra and Barak valley. Dhaka's plan worked well even after the British left as the ruling Indian National Congress found a ready vote bank in the Bangladeshi Muslims.
Its a pity that the Indian National Congress encouraged Dhaka's plan to send its population across the border and helped the illegal immigrants settel down by granting them Ration Card, Voters ID, Land Patta etc. The operation is simple Bangladeshis are allowed to enter Assam at will and settle down where ever they wish. All they have to do is talk to a local Congress official who gets the papers made to prove that the illigel immigrant is an Indian citizen. Since this has been happening for over a decade now, the network is far stronger than any India Government agency operating in Assam or for that matter the entire North East.
The irony is that Assam's former Home Minister is a Muslim, who is currently serving in the Tarun Gogoi led Congress ministry. During his former tenure as a Home Minister he made sure that the Assam Police had enough Bangladeshis to help the Muslims in any future event. As of today, there is hardly any police station in Assam that does not have a Muslim or Bangladeshi official. The case is same in all State Government Departments too, with many Muslims in top positions.
All this means that Assam has a Bangladeshi mole in every Police Station and every department. Even the Special Branch of the Assam Police is not left out. This means that in case of an Emergency the Indian Government can not trust any Government Department in Assam and has to deploy the Army pronto.
Currently there is hardly any corner in Assam that does not have a sizeable Bangladeshi presence. Dhaka can sieze Assam officialy any moment they like. The invasion of Assam that had started in the pre independence era was complete long back, all that remains now is an Official Declaration.
Is that what New Delhi is waiting for, is Assam not worth any concern? Well ofcourse New Delhi cares, but not for Assam but the number of votes that the Bangladeshis hold for being in India's driving seat.
There is only one word of caution for New Delhi, just as Assam has gone out of its hands today, the day would be not far when a Bangladeshi becomes the Prime Minister of India.
Not possible? Don't be fooled dear Indians, it took Bangladeshis less than 50 years to annex Assam so don't be surprised if your grand children are forced to change their religion and become Muslims.