After becoming emperor officially, Aurangzeb embarked upon an era of strong rule. In some regions, such as the north-east and the Deccan, the imperial frontier was advanced.
Aurangzeb’s first attempt immediately after his succession was to restate imperial authority and prestige, which included recovery of the regions, which had been lost during the war of succession and to which the Mughals felt that they had the legal claim.
The kingdom of Kamata (Kamrup) declined by the end of the fifteenth century and was replaced by the kingdom of Kuch (Cooch Bihar), which dominated north Bengal and western Assam and continued the policy of conflict with the Ahoms.
In 1612, the Mughals defeated and occupied the western Assam valley up to Bar Nadi with the help of Kuch armies.
The Kuch ruler became a Mughal vassal. Likewise, he Mughals came into contact with the Ahoms who ruled eastern Assam across the Bar Nadi.
After a long war with the Ahoms who had harbored a prince of the defeated dynasty, in 1638, a treaty was made with them, which fixed the Bar Nadi as the boundary between them and the Mughals. Thus Gauhati (Assam) came under Mughal control.
Mir Jumla, who had been appointed as the governor of Bengal by Aurangzeb, wanted to bring Cooch Bihar and the entire Assam under Mughal control.
Mir Jumla first attacked Cooch Bihar (which had rejected Mughal suzerainty) and annexed the entire kingdom to the Mughal empire. Next Jumla invaded on the Ahom kingdom and occupied its capital Garhgaon. Likewise, the Mughal boundary was extended from the Bar Nadi to the Bharali River.
Mir Jumla died soon after his victory. Later, the Ahom regained its power, which had not been broken, and also it was beyond the Mughal power to enforce the treaty.
In 1667, the Ahoms renewed the contest. They not only recovered the areas ceded to the Mughals, but also occupied Gauhati (Assam).
Over a period of time, the Mughal forces had also been expelled from Cooch Bihar. Likewise, all the won territories of Mir Jumla were rapidly lost. But later the shock of the Mughal invasion and the subsequent warfare damaged the strength of the Ahom kingdom and led to the decline and disintegration of the Ahom empire.
Shaista Khan succeeded Mir Jumla as the governor of Bengal after his death. He gave personal attention to the problem of south Bengal, where the Magh (Arakanese) pirates, in conjunction with Portuguese pirates, had been terrorizing the area up to Dacca (capital of Bengal) from their headquarters at Chittagong. The land up to Dacca had become deserted and trade and industry had suffered a setback.
Shaista Khan strategically built up a flotilla to meet the Arakanese pirates and captured the island of Sondip as a base of operations against Chittagong.
The Arakan navy near Chittagong was routed out and many of the ships captured. In 1666, Shaista Khan attacked Chittagong and captured. The destruction of Arakanese navy opened the seas for free trade and commerce.