It took place in AD 359 when the Sasanian army under king Shapur II invaded the eastern provinces of the Roman Empire.

Shapur wanted to exploit the absence of the Roman Emperor Constantius II who was overseeing affairs in the western part of the Empire.

[8] Following a prolonged struggle from (353-358) the Xionites were forced to conclude a peace, and their king, Grumbates, accompanied Shapur II in the war against the Romans.

[13] As news of the Persian invasion spread, the civilian population of the region began to panic: "Dispatch riders were sent at once to Cassian, the general of Mesopotamiam and Euphronius, then governor of the province, with orders to compel the country folk [farmers] to move with their families and all their livestock to places of safety.

Carrhae was to be evacuated immediately, because of the weakness of its fortifications, and the whole country set on fire (see:scorched earth), to deprive the enemy of a source of fodder.

[15] The Romans wanted to use the city as a base to strike at the Sassanids if they besieged Nisibis or to attack their lines of communication and supply if they moved westwards towards Syria.

The Sassanids suddenly marched to Amida, where their vanguard almost captured Ursicinus when he rode out with the cavalry wanting to make sure the bridges across the Euphrates were broken down.

According to Ammianus Marcellinus [1] The king himself [Shapur II], mounted upon a charger and overtopping the others, rode before the whole army, wearing in place of a diadem a golden image of a ram's head set with precious stones, distinguished too by a great retinue of men of the highest rank and of various nations.

[20] Ammianus Marcellinus also mentions that the king, whom he assumes to be Shapur, was called "Saansaan" and "Pirosen" by the Persians, which could actually refer to "Šāhanšāh Pērōz", the ruler of the eastern Hunnic tribes (Chionites, Gelani, and Sagistani).

Here, mingled with the Persians, who were rushing to the higher ground with the same effort as ourselves, we remained motionless until sunrise of the next day, so crowded together that the bodies of the slain, held upright by the throng, could nowhere find room to fall, and that in front of me a soldier with his head cut in two, and split into equal halves by a powerful sword stroke, was so pressed on all sides that he stood erect like a stump.The siege took 73 days.

Early in the siege a company of 70 elite Persian foot-archers by aid of a Roman renegade gained entrance to a tower on the south side of the city, which was placed on the bank of the Tigris.

Although a slight noise warned the Persians in time, the Gauls inflicted heavy casualties before retiring in good order within the walls.

Aside from having spent the campaign season in the reduction of a single city, Shapur II had lost as many as 30,000 in the siege, and his barbarian allies from the east deserted him due to the heavy casualties.