Aryans began to infiltrate into the Bengal region from the first millennium BCE. Bengal was then divided into many states and semi states dotted by autonomous tribes. The early Vedic Aryans did not make any contact with this part of the world for colonization on the ground that the region was unholy and inhabited by dasyus or outlandish barbarians. An account of the Matsya Purana goes that a blind old sage was by mistake drifted on a raft down the Ganges. A king called Bali had no children and he implored the sage to gift him children to inherit his kingdom. The sage's blessings, the king got five sons from his old wife. They were Anga, Vanga, Kalinga, Pundra and Suhma. Five countries of the blind king were named after those of his sons. These mythical kingdoms roughly correspond to Mughal Bengal and Behar.
The region was so unholy to the Aryans that they carefully refrained themselves from coming to this region and settle. The sage was blind and had no control over his raft floating on the Ganges waters. Hence, the blind Aryan's contact with this region was an accident. But the contact took place anyway and since then the Aryans began to come though very sparsely and casually. Interestingly, even the much later Vedic literature also depicted the people of Bengal as dasyus. 'The Mahabharata described the people of coastal Bengal as Mlechchhas. The Bhagavata Purana described the Bengal peoples as papa or sinful. The Dharmasastra prescribes expiatory rites after a contact with the Pundras and the Bangas. But as time passed Aryan infiltration into Bengal increased and so did their influence on the region. While the early Aryans accepted only the upper Ganges valley as their true home, the author of the Manava Dharmasastra extended it from the western to the eastern sea, though the law-givers branded the Paundras and Bangas as degraded. The Kshatryas, who made contact with them were treated as polluted too.
The Tirtha-yatra section of the Mahabharata made many spots along the Korotoya and the lower reaches of the Ganges holy for sinners to come and make them pure. It only indicates, Bengal was no longer an unholy place for the Aryans. But there is no reason to believe that the Aryans came to Bengal on any large scale ever. Those who came earlier made their new home habitable by making many spots holy for visiting. Thus Bengal entered into the Aryan culture, though the Upper Ganges Aryans did never recognise the lower riperian peoples fit for association. The Aryans lived in Bengal having maintaining a lot of restrictions about their interactions with the local people.
By 500 BCE, Bengal region witnessed the rise of several powerful states against whom the Aryan states of the Upper Ganges launched military campaigns. The Rama-epic records a story that the great Aryan heroes like Karna, Krishna and Bhimasena had battles with the powerful kings of Bengal and came out victorious. The story goes that the Aryans began to take the Bengal kings seriously from military point of view, at least. Krishna raised his status as a warrior by defeating the kings of Vangas and Pundras. Bhimsena, in the course of his eastern campaigns, subdued all the local princes of Bengal. All these epic stories are indicative of the change of impression of the Aryans about the peoples of Bengal and also of the beginning of the influence of the Aryan culture on the indigenous societies. [Sirajul Islam]