The Parthians were certainly not a threat to Rome at this point. Rome's great enemy in the east was prostrated by the after-effects of an enduring plague and from some very rough handling by the army of Septimius Severus some fifteen years previously. As is often the case, instability in Parthia had bred further instability, and the country was now riven by a bitter civil war between the brothers Artabanus V and Vologaneses VI. Consequently, Caracalla felt that his enemy was ripe for the picking. (Nor was he completely mistaken in this assessment. The Parthian empire was in its death throes and would fall within a decade. However, due to circumstances beyond Caracalla's control, the final blow would come from rebellion within rather than from the force of Roman arms.)
On his arrival in the east, Caracalla reviewed the available evidence and decided that the civil war in Parthia could safely be left to ruin the country further while he mustered his armies and waited to intervene. Therefore his early interventions in the east were limited to intelligence gathering and the opportunistic seizure of land around Edessa which became the short-lived province of Osroene. The Historia Augusta, an inaccurate and sensationally libellous account of Caracalla's life, describes this early phase of the campaign as 'guerilla warfare'.
As was almost obligatory with Romano-Parthian relations whenever one empire discerned weakness in the other, Caracalla attempted to seize control of Armenia. He did this by inviting the Armenian king to a conference, ostensibly to mediate in a quarrel between the king and his sons, and then took the king captive on his arrival. Much the same tactic had been used to take control of Edessa, but in this instance the tactic backfired. The Armenian people promptly rebelled, and defeated the Roman force sent against them. Thereafter they refused to believe or trust any Roman peace initiatives.
At the end of the campaigning season, Caracalla took himself to Alexandria in Egypt. His motives for going there are uncertain and are further obscured by hostile contemporary sources. Dio, for example, believes that he felt the city still honoured the memory of his brother Geta, and was determined to punish the Alexandrians for it. (Dio 78.22ff) A more probable scenario is that Caracalla rightly or wrongly believed that the city was disloyal and he did not want a rebellion at his back once his army was committed to the invasion of Parthia. Whatever the reason, it appears that Caracalla was made welcome by the citizens and in turn he appeared to accept their friendship cordially while he distributed troops about the city. At a signal, the army set about plundering the city and massacring the inhabitants.
This provides us with at least one reason for this atrocity – that Caracalla intended the wealth of one of the East's most prosperous cities to fund his Parthian campaign, and rumours of Alexandrian disloyalty had provided the excuse for doing so.
Since he also needed an excuse for invading Parthia, Caracalla wrote to king Artabanus (who was coming out on top in the civil war with his brother) and demanded the hand of the Parthian king's daughter in marriage. Artabanus saw no reason to give Caracalla a dynastic claim to his empire, and sensibly refused. Consequently, in 216 Caracalla launched his long-planned invasion.
This invasion went without a hitch, mainly because Artabanus refused to fight and instead pulled his army back to the shelter of the highlands beyond the River Tigris. This left Caracalla free to rampage through Mesopotamia where he captured the ancient city of Arbela. Caracalla then wrote back to the senate and, according to Dio, claimed victories over an enemy that he had in fact not even seen. The emperor made no attempt to hold his conquests but took his army back into Roman territory at the end of the campaigning season with the intention of resuming operations the following year.
Throughout his trip to the east, the superstitious Caracalla had made a point of visiting every major religious shrine along his route. Even while preparing to massacre the people of Alexandria, Caracalla had visited the city's famous shrine of the Serapaeum. He had earlier performed the rites to Asclepius at the God's temple in Pergamum. Before the start of the spring campaign for 217 Caracalla intended to travel from Edessa to visit the shrine of the moon-god at Carrhae. Confident in the goodwill of the soldiers, he took only a small escort with him.
Caracalla's superstition was also evident in his dealings with his Praetorian Prefect, a man called Macrinus. Two separate prophecies had informed Caracalla that Macrinus would succeed him in the very near future. The emperor had responded by giving Macrinus greater honours but less power. This may have warned Macrinus that the capricious Caracalla was planning his downfall, and so caused the Praetorian Prefect to add a man called Julius Martialis to the imperial retinue.
Martialis had a grudge against Caracalla. According to one source, Caracalla had put the man's brother to death on unsubstantiated charges. Another source claims that Martialis had not been promoted to the rank of centurion as he felt he deserved.
Accordingly, Martialis approached Caracalla as the emperor was relieving himself by the roadside on the return trip from Carrhae. His bodyguard had withdrawn slightly to give the emperor some privacy, and this gave the chance for Martialis to inflict a fatal stab wound, though the assassin was cut down seconds later.
Caracalla was 29 years old when he died. With him died any chance of Roman expansion in the east, as his successor Macrinus had to make a hasty peace with the Parthians in order to consolidate his position in Rome. Despite his numerous character flaws, Caracalla had given the empire two decades of stability which caused later generations to look back to his reign with a degree of nostalgia.