south into Parthia itself, taking the cities of Babylon, Seleucia and finally the capital of Ctesiphon in 116 AD. Then Trajan declared Mesopotamia a new province of the Empire. In the process, he also captured the great city of Susa in Khuzestan. Those months of 116 and 117 saw western Parthia as a client state of the Romans. Trajan originally planned to annex Parthia as part of the Roman Empire, but ultimately decided instead to place Parthamaspates on the throne as a Roman client, doing so in late 116 AD. After the sudden death of Trajan, his successor Hadrian decided that the Roman Empire was too much extended to the east and accepted in late 117 AD a Treaty with the Parthians in which renounced to nearly all the conquests of Trajan in the region. War over Armenia broke out again in 161 AD, when Vologases IV defeated the Romans there, captured Edessa and ravaged Syria. But soon, in 163 AD, a Roman counter-attack under Statius Priscus defeated the Parthians in Armenia and installed a favored candidate on the Armenian throne. The following year Avidius Cassius began an invasion of Mesopotamia, winning battles at Dura-Europos and Seleucia and sacking Ctesiphon in 165 AD. The Antonine plague, possibly of smallpox, which was sweeping Parthia at the time now spread to the Roman army, leading to their withdrawal: the plague killed nearly half of the population in the Italian peninsula (for some months there were nearly 3000 deaths each day in the city of Rome) and the Romans' legions in the eastern territories of their empire were hampered when large numbers of troops succumbed to the disease. With the plague the Roman Empire stopped to grow and started a process of slow disintegration in the next centuries, according to historians like Theodor Mommsen. However, under Caracalla, an interesting twist in Parthian relations occurred. After submitting a request to marry the daughter of Persian king Artabanus V (potentially allowing an heir to assume control of both empires) Caracalla massacred the diplomatic party sent to arrange the marriage and attempted an invasion of Persia in 216 AD. This was eventually unsuccessful and the Persians soon retaliated, inflicting heavy losses upon the Romans. The replacement of the Parthian Empire by that of the Sassanids in 226 CE, which was more stable and effectively organised, shifted the balance of power against the Romans. The neighboring rivaling