In c.110 CE, the Parthian king Osroes I intervened in Armenia, where his nephew Axidares (a son of Osroes' brother and Pacorus II) was king. Osroes replaced Axidares with Parthamasiris, another son of Pacorus. This created a complex situation, because under normal circumstances, the Roman emperor would crown the Armenian king, although he had to be a member from the Parthian Arsacid family. So, Parthamasiris was allowed to rule because he belonged to the right family, and was not allowed to rule because the Romans had not crowned him.
The Roman emperor Trajan (r.98-117) decided to intervene, which the Parthians had not unexpected. The Graeco-Roman historian Cassius Dio summarizes what Parthian envoys, trying to avoid war, said:
When Trajan had set out against the Parthians and got as far as Athens, an embassy from Osroes met him, asking for peace and proffering gifts. ... At the same time he asked that Armenia be given to Parthamasiris, who was likewise a son of Pacorus, and requested that the diadem be sent to him; for he had deposed Axidares, he said, inasmuch as he had been satisfactory neither to the Romans nor to the Parthians.note[Cassius Dio, Roman History 68.17.2; tr. Earnest Cary.]
In other words, Osroes recognized that Rome was to crown the king of Armenia, and he had only removed a bad ruler (Axidares), so that a better man (Parthamasiris) could occupy the throne. Trajan was not impressed. In 114, he invaded Armenia, accepted the surrender of Parthamasiris, and converted the kingdom into a Roman province.note[Cassius Dio, Roman History 68.19.] Later, Trajan ordered the execution of Parthamasiris.