The subjugation of Bengal, was the immediate task of Harshavardhana after he ascended the throne of Thaneswar. Harshavardhana`s military campaign against Sasanka in Bengal was led by the idea to take revenge on Sasanka and the recovery of his sister Rajyashri from the clasp of the enemy. The campaign and conquest of Bengal by Harshavardhana is known from the narratives of Bana. From the accounts of Bana it is known that while Harshavardhana was marching against Sasanka, he had received a proposal from the king Bhaskarvarmana of Kamrupa, for forming an alliance against Sasanka, the then king of Bengal. Harshavardhana readily courted the alliance of the king of Kamrupa. According to Dr. R.D. Bannerjee, Harsha and the Kamrupa king had formed a confederacy against Sasanka, because they had a common enmity factor, Sasanka. However after gaining fortification with the alliance of Kamrupa, Harshavardhana hastened to lead the campaign against Sasanka. On the way he came to know that his sister had been released and she had taken shelter in the Vindhya forest. Harsha right at that time ordered his minister Bhandi to lead the army and he himself entered the Vindhya forest in order to rescue his sister. However Harsha managed to rescue her and later joined his army on the banks of the Ganges. Though Bana`s account could not draw clear conclusion about the result of Harsha`s campaign against Sasanka, yet it is clear that in the course of his campaign against Sasanka, Harsha had occupied Kanauj. Though the conquest of Kanauj by Harshavardhana is a hypothetical theory, because different historians have provided various opinions, yet it is generally believed that Harshavardhana had actually occupied Kanauj in the course of his campaign against Sasanka in Bengal. Sasanka however had prepared a masterly retreat after the army of Harshavardhana advanced in Kanauj.

The hostility between Harsha and Sasanka did not end with Sasanka`s withdrawal from Kanauj. It had continued for a lengthy period of time. Since there are no specific evidences about the result of the war of Harsha against Sasanka, it is pretty difficult for historians to determine whether Harsha ever came in real conflict with Sasanka. According to the "Arya Manjusri Mula Kalpa", Harsha had marched against the capital of Sasanka, defeated him and forbade him to move out of the country. But according to Dr. R.C Majumdar, the records available from epigraphic evidences and the medieval Buddha chronicle are vague and obscure. It does not provide clear information about the battle. Later evidences proved that Sasanka had reigned over Gauda without any reduction of power, definitely upto 619 A.D. Thus it is concluded by later historians that Harsha`s campaign against Sasanka was futile. When the tide of Harsha`s invasion rolled back, Sasanka seemed to have recovered his position and reigned in undiminished glory till his death.

According to some scholars, the closing years of Sasanka`s reign was full of trouble and chaos. Harsha and Bhaskarvarmana conjointly had defeated him and had reduced him to a subordinate chief. This is endorsed by the "Midnapore inscription" of 629 A.D., where Sasanka did not use the title "Maharajadhiraja". As a result of this victory, Harsha had included Kanvasuvarna and part of Bengal within his Empire, which was originally within the territory of Sasanka. Dr. R.G. Basak has also suggested that Sasanka`s defeat was indicated by his debased gold coins and his lost of overlordship of Ganjam district. But Dr. R.C. Majumdar has refuted the theory that Sasanka`s powers were curtailed by Harshavardhana. He rather holds that Sasanka had ruled in undiminished glory till his death. During his lifetime Harsha could not project any successful campaign against him and it was probably after his death that Harsha had captured Magadha. Dr. Majumdar has based his theories on the evidences provided by the "Ganjam inscription". The inscription delineates that Sasanka had enjoyed full sway of his Empire, consisting of Bengal, South Bihar and Orissa. The accounts of Hiuen Tsang who had visited Magadha in 637 A.D., also corroborates the theory that Sasanka had ruled in undiminished glory till his death.

After the death of Sasanka, Harsha conquered the kingdom of his adversaries, consisting of the regions of Magadha, West Bengal, Orissa and Kangada. Harsha had conquered Orissa and Kangada and the remaining part of Sasanka`s dominion by 643 A.D. However there is still controversy among scholars that whether Harshavardhana had actually captured east Bengal or not. The other groups of historians have pointed out that though Harsha had conquered Bengal, he did not rule over the entire part. Harsha`s ally Bhaskarvarmana had received a portion of Bengal, comprising the northeastern part, lying between east of Bhagirathi and north of Padma.