Siege of Amida

The Siege of Amida took place when the Sassanids, under Shah Shapur II besieged the Roman city of Amida in 359.

In this battle Ammianus Marcellinus, a historian of Greek origin from Antioch, was a Roman army officer; he described the siege in his work (Res Gestae).

Persian
When Shapur II took control of the Sassanid Empire he sought to regain old territories previously lost to the Roman Empire. After crushing the Arabs in the south, he moved east to deal with nomadic forces, the most prominent being the Xionites. 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. In 358 the Romans had failed to dissuade Shapur from attacking Mesopotamia, so the next year Shapur decided to invade. Shapur started the western campaign in 359.

Roman
Emperor Constantius II had increasingly been doubting the loyalty of general Ursicinus. As a result, he did not give him command of the Roman forces in the East, and instead gave it to Sabinianus. As news of the Persian invasion spread, the civilian population of the region began to panic:

In the panic which followed, several Roman legions chaotically escaped the Persian advance to the safety of Amida. These included the Legio XXX Ulpia Victrix and the Legio X Fretensis.

Preliminaries of the Siege
It appears the Sassanid plan was to bypass difficult fortresses like Nisibis and to then march straight into Syria. When his forces approached Amida, the Sassanids were provoked into attacking the city. This came about when the son of Grumbates, while inspecting the defences of Amida, was shot and killed with an arrow shot by the city garrison. Ammianus described how the Grumbates, outraged at his son's death, demanded revenge from the Romans: he compares the death to that of Patroclus at Troy. The Sassanids began the attack with siege towers and attempted to take the city hastily, but were largely unsuccessful. Unable to gain a quick victory, Shapur had to commit to capturing Amida in order to appease his ally.

Siege
According to Ammianus Marcellinus

He continues with the account of how he reached the safety of the city just as the Sassanids were descending on the city:

The siege took 73 days. Shapur attempted to capture the city several times but every time it ended with disaster. Many times siege towers were set on fire by the Romans. During the siege, plague broke out in Amida but ended after ten days by a light rain. A day before capture of the city, Ammianus Marcellinus escaped to Melitene, and thence returned to Antioch.

Outcome
After capturing the city, Shapur II advanced further and took Singara and some other fortresses in the following years. In 363, Emperor Julian, at the head of a strong army, advanced to Ctesiphon, but was killed in a battle. His successor Jovian signed a treaty of peace, by which the districts on the Tigris and Nisibis (totalling five Roman provinces) were ceded to the Persians, and the Romans promised to interfere no more in Armenia.