Caracalla had been the last of his line. Since he was hated by the senate, he had not risked appointing an heir whom his enemies would immediately have rallied behind. Therefore his death left a power vacuum at the top. After some deliberation the army decided to back Macrinus simply because they could think of no-one better. The senate also accepted Macrinus as emperor, partly because Macrinus had always been deferential to that august body, and mostly because he was not Caracalla. Thus the lawyer from the provinces became Rome's first non-senatorial emperor.
Macrinus gave the army a large bonus (called a donative) to the army on his accession. It was essential to secure the goodwill of the soldiers, not least because the mother and aunts of the deceased Caracalla remained influential and deeply suspicious of the role Macrinus had played in the assassination. Following Caracalla's example, Macrinus gave the family honours – he even deified Caracalla – while moving to strip away their powers. Provincial governors whom Macrinus felt were too deeply attached to the Severan dynasty were ordered to stand down and replacements were sent to take their places.
Julia Domna, Caracalla's mother was ordered to leave Antioch, the local seat of power, and to return with her sister Julia Maesa to their home in Edessa. Julia Domna was already terminally ill – possibly from breast cancer – and she simply refused to go. It was the matriarch's final and very astute political manoeuvre. When Julia Domna did die soon afterwards, many held Macrinus responsible.
Meanwhile Macrinus' promising start with the army ran into difficulties. Caracalla had come east to campaign against the Parthians and this war continued. Macrinus had no military experience and he was a fussy perfectionist who punished mistakes more than he rewarded success. Predictably, this meant that when the Romans did meet the Parthians in a major battle near Nisibis in 217, Macrinus' commanders were more intent on avoiding defeat than in securing victory. The battle was a draw, though the enemies of Macrinus preferred to paint it as a defeat, comparing it unfavourably with the run of successes enjoyed by Septimius Severus against the same enemy.
Macrinus knew his own limitations, and was anyway prepared to cut his losses and get out of the Parthian war. He needed to stamp his personal authority on Rome, where the populace both disliked his appointee as city prefect and the implied insult of an emperor who appeared uninterested in returning to the city. The Parthian king had his own problems, including a simmering civil war. He cheerfully accepted Macrinus' offer of peace in exchange for a huge cash payment, and in return allowed Macrinus to place his choice of client king upon the throne of Armenia. (It is uncertain that this cash payment was ever made.) None of this sat well with the army. By some accounts it appears that Macrinus, ever the conscientious administrator, decided to recoup some of his financial losses from military wages. He was not so foolish as to cut the pay of serving soldiers, but he lowered the starting salary for new recruits.
The new emperor may have felt that this cut was compensated by a financial reform – again as a conscientious administrator, he would have noted that rising inflation had as its root cause the ever-declining amount of silver in the standard Roman coin, the denarius. (Once pure sliver, the silver content of the coin was now around 51%.) Macrinus slightly increased the silver content of the coin and may have hoped that the increased purchasing power compensated in part for the cut in recruits' pay. The army instead noted only an unsuccessful battle, peace without victory (and more importantly, without booty) and a pay cut.
All this played into the hands of Macrinus' enemies, especially the sister of Julia Domna. This was Julia Maesa, aunt of the late Caracalla, and a lady dedicated to restoring the dynasty of Severus, and her own position as a power behind the scenes. Julia Maesa had a grandson, a handsome youth called Varius Avitus Bassianus. Julia started a rumour that this fourteen-year-old boy was in fact the natural son of Caracalla, thus giving the youth that crucial family tie to the Severan dynasty which Macrinus lacked. Macrinus had tried to make the family connection in his titles, calling himself Severus and his son Antoninus, but this move was widely mocked.
Once the rumour of his parentage was well-established, Julia smuggled the boy into the camp of III Gallica, a legion based at Raphanaea at a strategic location between Antioch and Damascus. She had taken the precaution of further boosting the legion's pro-Severan sympathies with a large cash payment, and on 16 May the legion became the first to declare for the boy emperor who was to later be known as Elagabalus after the sun god whom he worshipped.
Macrinus responded by sending his Praetorian prefect and the legion II Parthica to suppress the revolt. He also promoted his own son 'Antoninus' Diadumenianus to the rank of Caesar. It will be remembered that Macrinus had commanded II Parthica a few years previously, so it says something of his powers of leadership that his soldiers killed the Prefect and promptly joined III Gallica in rebellion.
With the rebel army advancing on Antioch and more defections occurring daily, Macrinus had to make a stand. On 8 June 218 he met the rebels in battle, and despite having more experienced troops and superior commanders ,his unmotivated and demoralized army was soundly defeated.
Macrinus had already sent his son off to seek safety in Parthia under the guise of an embassy. He knew that his only chance at survival was to escape to the west and attempt to rally the legions there. Shaving his head and beard Macrinus set off westward disguised as a courier - an interesting choice of disguise given that he was allegedly a courier in his youth. He got as far as Chalcedon on the coast of Bithynia before he was arrested 'like a common thief'. His son had also failed to escape, and the pair were executed.
Macrinus was solid and capable, but he lacked the personality, family connections and flair that citizens and soldiers looked for in a ruler.