Although peace with Hormuzan had been accepted for the second time, it proved to be shortlived. Hormuzan was smarting under the disgrace of repeated defeats, and the loss of a greater part of his princedom. He had lost Ahwaz which was the main city of his dominions, and he held his court at Ram Hormuz which was a small mofussil town to the east of Ahwaz.
After the conquest of Ctesiphon the capital of Persia by the Muslims, the emperor Yezdjurd shifted to Hulwan. He had hoped that the Persian forces concentrated at Jalaula would hold up further advance of the Muslims in Persia. These hopes were not realised and the Persians suffered another defeat at Jalaula. That made Hulwan unsafe for the emperor and he fled to Qum. Khaniqeen and Hulwan were captured by the Muslims the emperor fled to Kashan and then to Isfahan. The emperor became a fugitive in his own kingdom and the court had to move from town to town. The Persian administration collapsed to such an extent that the Persians had no capital, and very little of governmental organisation.
Yazdjurd, however, continued to make strenuous efforts to rally the Persians for another confrontation with the Muslims. He appealed to the Persians in the name of their religion and their country to line up for another major effort to defend their homeland. The Persians assured him that they would stand by him. At this juncture the emperor felt that Hormuzan could, suitably act as the vanguard or Persian resistance against the Muslims. Yazdjurd appealed to the patriotism of Hormuzan and prevailed upon him to spearhead the Persian struggle against the Muslims. The emperor assured him of full support.
Hormuzan accordingly undertook to make another effort to drive away the Muslims from the Persian soil. Hormuzan was thus once again on the war path. He built a strong force. The emperor placed the resources of Persia at his disposal. The Persians took the oath by the sacred fire that they would win or die.
The war preparations in the Persian camp were reported to Abu Musa the Governor of Basra. Abu Musa reported to Umar that trouble could be expected from Hormnzan any moment. Umar ordered that before Hormuzan should gather further strength the Muslim forces should advance against him, and take him to task for breaking his pledges repeatedly. Umar also wrote to the Governor of Kufa that a column should be sent from Kufa to reinforce the Muslim forces in the Basra sector.
The action began with the advance of a Muslim column under Noman bin Muqarrin from Ahwaz to Ram Hormuz. The Muslim force crossed the river near Ram Hormuz at Arbuk and confronted the Persian force arrayed on the east bank. A sharp engagement followed which resulted in the fight of the Persian force from the battle-field. Hormuzan with his army left Ram Hormuz undefended and retired to Tustar north of Ahwaz.
The Muslims occupied Ram Hormuz and the residents surrendered on the usual terms. After leaving a garrison at Ram Hormuz the Muslim force marched northward to Izaj at the base of the Zagros mountain. That was the easternmost district of the province of Khuzistan. No reistance was offered and Izaj was occupied by the Muslims.
Tustar which Hormuzan had occupied lay west of Izaj. Intelligence was brought that Hormuzan had fortified himself at Tustar. Abu Musa felt that the Muslim forces should not march to Tustar unless these were further reinforced. Leaving a garrisonat Izaj the main Muslim force returned to Ahwaz. A detailed report was submitted to Umar and his further instructions were sought.
Yazdjurd had sent some forces for the help of Hormuzan. When Hormuzan was defeated at Arbuk and fled to Tustar, a contingent of the Persian force under General Siyah crossed over to the Muslim camp and accepted Islam. That was a welcome addition to the Muslim force.
Umar wrote to Abu Musa that he was sending help and that when he was reinforced he should march to Tustar. Umar asked Ammar bin Yasir the Governor of Kufa to despatch a detachment from Kufa to augment the stregth of Abu Musa's army. Ammar b. Yasir despatched a force of 1000 men under Jareer b. Abdullah. In compliance with further instructions from Umar, Ammar himself marched with half of his army to the aid of Abu Musa. Ammar left Abdullah b. Masud as his deputy at Kufa.
After having received reinforcements, Abu Musa decided to launch the attack against Tustar. In 610 AD, a Muslim column under Noman bin Muqarrin marched from Ram Hormuz to Tustar. The rest of the Muslim army including the contingents from Kufa met at Ahwaz and from there marched to Tustar.
Tustar lay to the north of Ahwaz upstream the Karun. The Muslim forces marched through the Karun valley and without any encounter reached Tustar.
Tustar was a walled city with battlements in the walls. Inside the city there was a strong citadel. The town had its water supply from a canal from the Karun river. Outside the city, Hormuzan had a deep ditch dug. Ihe town was stocked with provisions adequate to last for a year. A large Persian force was quartered inside the city. As Hormuzan surveyed the arrangements made for the defence of Tustar he felt assured that the city was unassailable and invulnerable. Hormuzan felt strong enough to fight in the open. His strategy was to drive away the Muslims before they could settle to a regular siege.
As soon as the Muslim forces arrived at Tustar, Hormuzan challenged them to action. The two forces met in the plain south east of Tustar. The Persians fought desperately, and for some time they appeared to have the initiative. Then the Muslims charged heavily and the Persians were forced to withdraw to the safety of the ditch.
Thereafter the Muslims besieged the city. Detachments of Muslim forces were stationed at key points, and all routes of access and escape for the Persians were closed.
The siege dragged on for several months. There were skirmishes every now and then, but these were not conclusive, and no side could claim success. After some months the Persians made a desperate sally. In the fierce fight that followed the Persians lost ground and hastily withdrew. The Muslims followed close on their heels and were able to capture the ditch. The Persians having lost the protection of the ditch shut themselves in the walled city.
The Muslims now closed round the walls of the city with the ditch at their command. With the tightening of the siege, the Persians within the city got demoralised. There were dissensions among the Persians. A Persian Seena by name escaped from the city, and waiting upon the Muslim Commander offered to show the Muslims an easy way to capture the city. The offer was accepted and the Persian accepted Islam. One night, Seena led a band of Muslim warriors inside the city through the main sewer. The guards at the main gate of the city were overpowered, and the gate was thrown open for the Muslim force to enter.
The Persians were taken by surprise, but they nevertheless put up a stiff fight. With sword in hand, Hormuzan fought desperately. He killed two eminent companions Baraa bin Malik and Majza'a bin Saur. As the Muslim forces increased heir pressure, the Persians withdrew to the citadel. Now the city was in the hands of the Muslims, but Hormuzan and his forces were in the fort. The Muslims besieged the fort. The residents of the city deprived of the protection of the Persian army surrendered.
The following day Hormuzan hoisted the flag of peace on e citadel. He mounted the roof of the citadel and said that was prepared to surrender on the condition that Umar himself decided the case. The offer was accepted, and the Persians formally surrendered. The Muslims were now the masters of Tustar.
The booty was collected and distributed. Each cavalryman received a share of 3,000 dirhans while a footman had a share of 1,000 dirhams. The usual one fifth state share of the booty was despatched to Madina. Hormuzan was sent under escort to Madina for the decision of his case by Umar.