Macrinus now sends letters to the Senate denouncing Elagabalus as the False Antoninus and claiming he is insane.

Both consuls and other high ranking members of Rome's leadership condemn him, and the Senate subsequently declares war on both Elagabalus and Julia Maesa.

A force under Bassianus’s tutor Gannys marches on Antioch and engages a force under Macrinus Macrinus and his son, who, weakened by the desertion of the Second Legion due to bribes and promises circulated by Julia Maesa, are defeated on June 8, 218 at the Battle of Antioch.

Macrinus flees toward Italy, disguised as a courier, but is later intercepted near Chalcedon and executed in Cappadocia.

His son Diadumenianus, sent for safety to the Parthian court, is captured at Zeugma and also put to death.

The demise of Macrinus has reinforced the notion of the soldiers as the true brokers of power in the third-century empire and highlighted the importance of maintaining the support of this vital faction.

Elagabalus declares the date of the victory at Antioch to be the beginning of his reign and assumes the imperial titles without prior senatorial approval, which violates tradition but is a common practice among third-century emperors nonetheless.

Letters of reconciliation are dispatched to Rome extending amnesty to the Senate and recognizing the laws, while also condemning the administration of Macrinus and his son.

The senators respond by acknowledging Elagabalus as emperor and accepting his claim to be the son of Caracalla.

Caracalla and Julia Domna are both deified by the Senate, both Julia Maesa and Julia Soaemias are elevated to the rank of Augustae, and the memory of Macrinus and Diadumenianus is condemned and vilified by the Senate.

The former commander of the Third Legion, Comazon, is appointed to be commander of the Praetorian Guard.