The Mughal path to Assam ran through the ruins of the Koch dynasty. When the Koch kingdom split in 1581 into Koch Bihar in the west and Koch Hajo in the east, a bitter rivalry between the two branches created a power vacuum that the Mughals exploited methodically. Koch Bihar became a Mughal vassal. Koch Hajo aligned first with Bengali warlords, then with the Ahoms, but the Mughals marched against it anyway, removing its ruler in 1613 and establishing four administrative divisions in the newly acquired territory. The local population, governed for generations under the Ahom paik labor system, found themselves suddenly subject to Mughal revenue demands. Revolt followed. For decades, Ahom kings sponsored anti-Mughal resistance in the region, offering refuge to dissidents and military aid to rebel chieftains, probing for weakness without committing to open war.

Mir Jumla was a shrewd tactician. Before reaching Assam, he first annexed Koch Bihar in a rapid campaign, entering its capital unopposed on December 19, 1661, after outmaneuvering its defenses by discovering an unguarded route through a low embankment that bypassed the fortified passes. He renamed the city Alamgirnagar, stationed a garrison, and pressed east. His invasion of Ahom territory began with a bold stroke: on February 15 and 16, 1662, he moved his entire army across the Brahmaputra to the south bank at Borsola, a crossing that threw the Ahom command structure into confusion. The Ahom generals, expecting an advance along the north bank, scrambled to reposition their forces. Internal betrayal compounded the chaos. The disgraced former Borphukan, bitter at losing his post, guided Mughal forces through the plains of Nagaon. One by one, Ahom forts fell or were abandoned.

When Mir Jumla entered Garhgaon, the Ahom king had already fled eastward. The spoils were staggering: 675 large guns, over 6,500 matchlocks, nearly 8,000 iron shields, roughly twelve tons of gunpowder, and a thousand warships seized from the Ahom dockyards at Lakhau and Trimohani. Mir Jumla pillaged temples, including the sacred Kamakhya Temple, and broke open the royal moidams, the burial mounds of past Ahom kings, recovering goods worth some 90,000 rupees. He treated all of it as imperial property and dispatched the treasure to Dhaka. But occupying Garhgaon proved far easier than holding it. When the monsoon arrived, it brought not just rain but catastrophe. The Brahmaputra flooded the lowlands, severing Mughal supply lines and isolating garrisons. Disease, likely malaria and dysentery, devastated the occupying army. Ahom guerrilla forces harassed the weakened Mughals from every direction.

By the time the monsoon receded, Mir Jumla commanded an army more desperate than victorious. The Ahoms, themselves exhausted and weakened by noble defections, sued for peace. The Treaty of Ghilajharighat, concluded in January 1663, required the Ahoms to pay twenty thousand tolas of gold, six times that amount in silver, and forty elephants immediately, with further payments and hostages to follow. The Ahoms ceded territory west of the Bharasi River on the north bank and the Kallang River on the south. It was a face-saving arrangement for both sides. Mir Jumla began his withdrawal on January 10, 1663, but he never made it home. He died en route to Dhaka, likely from the same diseases that had ravaged his troops. Within four years, the Ahoms pushed Mughal garrisons out of Assam entirely, retaking control as far west as the Manas River by December 1667. The invasion that had once seemed a conquest became, in the longer view, a costly and humiliating Mughal overreach.