By the early third century, the balance of power had shifted from the Senate to the army, and the position of the Senate was considerably weakened. The emperor of Rome was appointed by the support of the military, while the Senate existed solely to officiate state affairs without any real authority. Both Macrinus and later Elagabalus secured the support of the military while generally disregarding the opinion of the Senate. Macrinus was in dire circumstances after Elagabalus’ rebellion and had no other choice but to turn to the Senate for assistance. While in Antioch, Macrinus made one final attempt at securing support, this time from Rome. A combination of distrust from the Senate, insufficient funds, and Elagabalus’ impending approach, however, forced Macrinus to face Elagabalus’ approaching legions with only his Praetorian Guard. Had more time had been available, the Urban Prefect of Rome, Marius Maximus, might have been able to muster troops to send as reinforcements to assist Macrinus. Despite their relative powerlessness, the Senate still declared war against the usurper and his family.
Descriptions of the battle differ, and its location is debated. The decisive and perhaps sole engagement took place on 8 June 218; Dio places it at a defile outside of a village believed to be Immae, approximately twenty-four miles or so by road between Antioch and Beroea. Herodian challenges this assertion, suggesting that the battle took place closer to the borders of Syria Coele and Syria Phoenice, possibly near Emesa. Downey then suggests that two battles took place: an initial engagement matching the one described by Herodian, and a later battle near Antioch, which Downey agrees was the decisive point in the rebellion. Other historians either support Dio’s suggested site near Antioch or make no claim with regards to the location of the engagement.
Elagabalus’ armies, commanded by the inexperienced but determined Gannys, engaged Macrinus’ Praetorian Guard in a narrowly fought pitched battle. Gannys commanded at least two full legions and held numerical superiority over the fewer levies that Macrinus had been able to raise. Nonetheless, the engagement began in Macrinus’ favour. According to Dio, Macrinus had ordered the Praetorian Guard to set aside their scale armour breastplates and grooved shields in favour of lighter oval shields prior to the battle. This made them lighter and more manoeuvrable and negated any advantage the legionary lanciarii (javelin-armed light infantry) had. The Praetorian Guards broke through the lines of Gannys’ force, which turned to flee. During the retreat, however, Julia Maesa and Soaemias Bassiana (Elagabalus’ mother) joined the fray to rally the forces while Gannys charged on horseback headlong into the enemy. These actions effectively ended the retreat; the troops resumed the assault with renewed morale, turning the tide of battle. Fearing defeat, Macrinus fled back to the city of Antioch. Historians suggest that had Macrinus not fled, he might have eventually gained victory and secured his position as emperor.
After his defeat, Macrinus sent his son Diadumenian to Artabanus V of Parthia, while he himself returned to Antioch, proclaiming himself victorious over Elagabalus in battle. News of Macrinus’ defeat spread and many civilians who had supported him were slain in the city and on the roads. Macrinus shaved off his beard and hair to disguise himself as a member of the military police. Fleeing the city at night on horseback, he reached Cilicia with a few companions, masquerading as a military courier, and secured a carriage to drive to Eribolon, near Nicomedia. From there he set sail for Chalcedon.
Macrinus travelled through Cappadocia, Galatia and Bithynia before arriving in Chalcedon. Here he was arrested, his guise revealed after he had sent requests for money. Men dispatched by Elagabalus apprehended Macrinus and brought him to Cappadocia. Diadumenian was captured elsewhere on his journey to Parthia, and killed by the centurion Cladius Pollio in Zeugma. French author Jean-Baptiste Crevier comments that Macrinus threw himself out of the carriage at Cappadocia after receiving the news of the death of his son, breaking his shoulder in the act. Macrinus was executed in Archelais in Cappadocia after attempting to escape; Dio mentions that the centurion Marcianus Taurus was responsible for his execution. Thus, the reign of Macrinus as emperor of Rome ended after nearly fourteen months.
In the meantime, Elagabalus had entered Antioch and declared himself the new ruler of Rome in a message to the Roman Senate and people. Once again, as they had done with Macrinus, the Senate were forced to recognize Elagabalus as the new emperor. Elagabalus’ claim was not uncontested, as several others made their own bids for the imperial purple. These included Verus, the commander of Legio III Gallica and Gellius Maximus, the commander of Legio IV Scythica. History professor and author Martijn Icks notes the irony of Verus’ claim as his legion had been the first to proclaim Elagabalus as the rightful emperor of Rome. These rebellions were quashed and their instigators executed. By March 222 A.D., Elagabalus was himself murdered by the Praetorian Guard, his body dumped in the river Tiber and his memory condemned by damnatio memoriae ordered by the senate.