After the conquest of Jalaula in December 637 AD, Hashim sent a contingent under Qaqaa b. Amr to pursue the Persians to Hulwan. Hashim left a garrison of 4,000 men under Jareer b. Abdullah at Jalaula to guard against hostile moves from the north. With the remainder of the army, Hashim returned to Ctesiphon.
In spite of the fall of Ctesiphon and Jalaula, there were some Persian pockets upstream the Tigris and the Euphrates. After the fall of Jalaula, Saad undertook campaigns in the Tigris valley at Takreet and Mosul.
For the Euphrates valley, Saad organised an expeditionary force and sent it under the command of Amr b. Malik to deal with the Persian pockets at Heet and Qirqassia.
In the last week of December 637 AD, the Muslim forces under Amr b. Malik arrived at Heet. Here they found that the Persians had dug a ditch round the town. Amr pitched his tents beyond the ditch, and decided to lay siege to the town.
The Persians remained shut in the town and remained on the defensive. Further upstream at Qirqassia at the junction of the Khabur and the Euphrates there was another Persian cantonment. The Persians at Heet continued to receive provisions from Qirqassia through the river.
Amr b. Malik waited at Heet for some time, but no engagement took place. Amr thought that in order to force the Persians at Heet into submission, their source of supply from Qirqassia should be cut.
Amr decided to pounce upon Qirqassia, take them unawares and thereby cut the source of supply to Heet. Amr left the Muslim camp at Heet standing occupied by a detachment under Haris b. Yazeed. With the rest of the army Amr marched off at night for Qirqassia. The Muslim force appeared suddenly, at Qirqassia. The Persians offered some feeble resistance, but being no match for the Muslims the Persian garrison surrendered. The representatives of the inhabitants of Qirqassia waited on Amr and sought for terms. The usual alternatives of Islam or Jizya were offered and the inhabitants agreed to pay Jizya.
With the surrender of Qirqassia thc source of supply to Heet was completely cut. Amr sent a fast courier to Haris at Heet instructing him to inform the defenders of Heet that Qirqassia had submitted and agreed to pay Jizya. If the people of Heet agreed to submit on similar terms their safety lay in such a course. Amr further instructed Haris that if the people of Heet did not surrender he should construct another ditch outside the ditch of the Persians. In that case he would be returning from Qirqassia to resume the offensive against Hect.
The people of Heet came to know of the fall of Qirqassia from their own resources as well, and they felt that after the fall of Qirqassia they could not hold long against the Muslims. The people of Heet were mostly Christian Arabs, and they felt that the Persian forces were no longer strong enough to protect them.
When Haris called upon the people of Heet to surrender on the usual terms they accepted the offer and agreed to pay Jizya. In the second week of January the Persian forces withdrew from Heet. The inhabitants of the town opened the gates of the town to the Muslims. Thereupon the Muslim forces marched in and occupied the town.
With the occupation of Qirqassia and Heet, the Muslim hold in the Euphrates valley became firm. After restoring order Amr left garrisons at Qirqassia and Heet and himself returned to Saad at Ctesiphon.