[3] After intervening to overthrow rulers in client kingdoms adjoining Parthia, he invaded in 216 using an abortive wedding proposal to the Parthian king Artabanus's daughter as a casus belli.

While Artabanus eventually gained the upper hand, though without totally defeating his brother, the conflict destabilised the neighbouring kingdoms of Armenia and Osroene in the buffer zone between the Roman and Parthian Empires.

Caracalla exploited civil strife in both kingdoms in order to expand Roman power in the region and set the scene for an advance into Parthia.

[5] Caracalla travelled to the eastern Mediterranean in 215 and remained in the region for the rest of his reign, making Antioch his de facto capital during this period.

Dio and Herodian both report that Caracalla travelled to Alexandria in Egypt to pay his respects at the tomb of the Macedonian king but instead carried out a great massacre of the local population in 215.

According to Dio, Caracalla sought a pretext for war in the refusal by the Parthian king Vologases VI to release a pair of hostages – Tiridates of Armenia and a Cynic philosopher named Antiochus.

Both historians record that Caracalla justified his war on the grounds that Artabanus had denied the emperor's request to marry the Parthian king's daughter.

[6] Cassius Dio describes how Caracalla now ravaged a large section of the country around Media by making a sudden incursion, sacked many fortresses, won over Arbela, dug open the royal tombs of the Parthians, and scattered the bones about.