The Muslim Arab army in Persia, supposedly outnumbered five to one, feigns defeat on the battlefield, withdraws with the Persians in hot pursuit, surprises them amid two mountain passes, and decisively defeats them in 642 at the Battle of Nahavand, killing about one hundred thousand.

Various versions are told about Nahāvand and how the battle was ensued in the early stages.

Some note that the Muslim Arabs, under Sa`d ibn Abi Waqqas, managed to deceive the Persians through a ruse, that Caliph Umar had died.

The Persian cavalry, full of confidence, mounted an ill-prepared pursuit of the Bedouins who swiftly retreated to a safe area and eventually surrounded and trapped the Persian force before assailing it from all sides, and decisively defeating it.

As the historian Tabari mentions, the Persians were never again able to unite their men in such numbers and many were already talking of dissolving the Empire and going their separate ways when the battle was commencing.

Many of Yazdegerd's military and civilian officials have abandoned him.

Nahāvand marks the dissolution of the Sassanian Imperial army, with the fall of the last of the grand marshals of the army and the rise of warlordism among the Persians.

Yazdegerd attempts to raise troops by appealing to other neighboring areas, such as the princes of Tukharistan and Sogdia, and will eventually send his son Peroz III to the Tang court, but without any success.

The Muslim Arabs follow their victory at Navahand with the conquest of Isfahan, located on the north bank of the Zayandeh River (about two hundred miles/three hundred and twenty kilometers south of present Tehran), and make it a provincial capital.

Constantinople makes several attempts to retake Alexandria.

None of these are successful for a sustained period of time, but imperial forces in 645 are able to briefly regain control of the city, when Uthman dismisses Amr as military governor of Alexandria.

Constans, taking advantage of the power vacuum, dispatches a fleet to besiege the city, successfully at first, as Alexandria revolts from Arab rule at the appearance of three hundred ships of the imperial fleet, whose forces recapture the city.

Arab chroniclers tell of a massive fleet and army sent by Constantinople with the goal of retaking Alexandria.

The imperial forces are led by a lower ranking imperial official named Manuel.

After entering the city without facing much resistance, the imperial forces are able to regain control of both Alexandria and the surrounding Egyptian countryside.

The Muslims retaliate by readying a large force of fifteen thousand, who promptly set out to retake Alexandria under command of the veteran Amr ibn Al-As.

The imperial troops, following their standard tactical doctrine, advance out of the city and seek an open battle away from the shelter of their fortifications near the fortified town of Nikiou.

Accounts of the battle portray the Muslim forces as relying heavily on their archers before eventually assaulting the imperial positions, driving many back and routing the rest in the process.

After this, the imperial forces are utterly defeated and withdraw from the region.

The defeat of Manuel's forces marks the last attempt by the Empire to recapture Egypt for some five hundred years, with only Emperor Manuel I Komnenos sending an expedition here in the twelfth century.

Amr ibn al-'As builds fortifications in Alexandria and quarters in the vicinity a strong garrison, which twice a year is to be relieved from Upper Egypt.

An embassy from the Rashidun Caliphate led by Saad ibn Abi Waqqas arrives in the Tang capital Chang'an via an overseas route.

They are greeted by emperor Gaozong, who orders the establishment of the first Chinese mosque.