About the same time Mughal emperor Aurangzeb had presented a khelat or robe of honour to Supangmung, and the imperial envoys insisted on the king's donning them in their presence.
In March 1665 the king summoned an assembly of his ministers and nobles and ordered them to devise and adopt measures for expelling Mughals from Western Assam, adding--"My ancestors were never subordinate to any other people; and I for myself cannot remain under the vassalage of any foreign power.
The necessary preparation were made with all speed, and in August 1667 a well-equipped army set out led by Lachit Borphukan to wrest Guwahati from the Mughals.
King Supangmung gave valuable gifts to the commanders of the expedition and delivered to them the following message--"I desire that your wives and children, and the cows and Brahmans should be duly protected and preserved; and I should also acquire the prestige and reputation of having vanquished the Mughals.
In June 1664, he wrote to Raja Prana Narayan of Koch Behar, "You know for yourself all about the manner in which we repeatedly dealt heavy blows upon the Mughals.
A wrong man would have behaved differently, but the king's nominee was imbued with the same zeal and patriotism of the spirited sovereign, and he succeeded in expelling the Mughal from Assam.
Supangmung also ordered his Prime Minister Atan Burhagohain, a skilled soldier, military engineer and diplomat to accompany Lachit Borphukan on the campaign to oust the Mughals from Assam.
Sitting at the capital for nearly two years he managed and gave logistic support to the army in the front supplying men, arms and food materials.
From the capital he maintained an efficient civil administration in the country which was necessary to guarantee the regular reinforcements of men and materials to the Army in the frontline.