The Banu Lakhm were an Arab tribe who had founded a kingdom on the southern banks of the Euphrates, with its capital at Al-Hira. For a long time vassals of Persia, in 602 they had unsuccessfully revolted, resulting in the destruction of the kingdom and its annexation to Persia. Many Lakhmid Arabs however remained in the area, chafing under Persian rule, and Ali thought that he could incite them to rebel and join with the Caliphate.
Outbreak of War
In 634 Ali sent al-Mundhir, son of the last Lakhmid king, with a handpicked force of Muslim warriors to try winning over the tribal chiefs. The Lakhmids quickly rose up and killed their Persian governor, before swearing allegiance to Ali. However, the rebellion faltered once a Persian army led by Hormuzan arrived in the area and began to take reprisals against the locals.
Ali now sent most of his own army under Khalid ibn al-Walid north to rescue the Lakhmids. Though Khalid was able to fight Hormuzan to a standstill, their armies were too evenly matched for Khalid to be able to send any reinforcements once news of a second Persian army landing in Bahrain arrived.
Much of coastal Arabia, including Bahrain, Muscat and Yemen had formerly been subject to Persia, and Yazdegerd III wished to take advantage of the end of the Roman war to recover its dominance in the region. After subduing Bahrain, therefore, he split the army - one part under General Shahryar remained in the area to complete the subjugation and to protect supply lines, while the other part led by Rostam Farrokhzad began marching around the coast, aiming for the Hejaz. Yazdegerd himself accompanied Rostam, though as a child he had no military role to play.
Caliph Ali led his remaining troops against Rostam, but was unable to do more than harrass the flanks. Khalid and the bulk of the Caliphate's troops were still occupied in the north. As the Persians approached Himyar he called desperately upon his allies, King Wasan Sagad of Axum and Emperor Heraclius of the Romans, for assistance. Both agreed to help, but warned that it may be some time in coming.
In September 637 Wasan Sagad confronted the Persian army at the Battle of Abyan but suffered a heavy defeat. The Axumites retreated back across the Erythrean Sea, and the Persians continued on unopposed.
By April 638 the Persians had reached Mecca and were beginning a siege, but were interrupted soon after by disastrous news. Their supply fleet had been attacked and captured by the Axumite navy, and without it the Persians had no means of survival in the arid Hejaz. At the same time, scouts reported that a Roman army was fast approaching, and Rostam Farrokhzad decided that he had no alternative but to try to retreat.
At the Battle of Arafat on the 12th May 638, the Persian army was utterly destroyed by the Romans and Ali's few troops. Rostam Farrokhzad and most of his generals were killed, and young King Yazdegerd was captured. The result was a devastating blow for Persian might from which it would take a generation to recover.
Over the next few years, the allies struck back. Ali recovered all the lost lands in eastern and southern Arabia, while Heraclius took the opportunity to conquer most of Mesopotamia. Wasan Sagad took advantage of the crippling of Persian naval power to take over the maritime trade routes between India and Europe, greatly enriching his country.
Having proven his strength against a foreign enemy, at first Ali wanted to invade Persia itself and conquer new lands. He backed down however when Heraclius made it clear he would not have Roman support - the Romans were still recovering from the disaster that came of Justinian's reconquest of the relatively small Italian peninsula. As Ali's forces had only played a minor role in the final victory of the war, he was in no position to argue.
Finally, in 640 the war ended with a peace treaty in which the new king, Shahryar, swore never again to trespass on Roman, Arab or Axumite interests.