What was the impact of Commodus on Rome
Emperor Commodus (161 -192 AD) was one of the most despicable rulers in Rome’s long and violent history. He has been portrayed in many popular movies and tv series. Joaquin Phoenix even played a fictionalized version of Commodus in the movie Gladiator. Commodus's reign was a complete disaster.
His cruelty was legendary and comparable to Nero and Caligula. On a positive note, he did successfully end the Marcomanni Wars. Still, Commodus gross mismanagement and madness led to instability and a civil war in the short term and the longer term, undermined the Imperial system.
The era of the ‘Five Good Emperors’ is often seen as the high point of Rome and Roman civilization. Gibbon claimed that the period of their rule was the happiest period in human existence. The Roman Empire was largely peaceful and well-governed. The provinces had become more urbanized and Romanized, and a host of religions and minorities generally lived in peaceful harmony. The economy was generally buoyant, and long-distance trade flourished. Foreign invaders rarely breached the frontiers of the Empire.
The German tribes were an occasional treat, but the Parthians in the east had been humbled and weakened by Trajan. However, this era was not quite the golden age as depicted by Gibbon. There was a great deal of poverty, inequality, banditry, and rebellions were not unknown. The Macromannic War, when Marcus Aurelius after many hard battles defeated a powerful confederation of German tribes, was an indication that the Romans were not invincible. Moreover, the so-called Antonine Plague had decimated the population of the Empire, and this demographic disaster was to have long-term consequences for Rome. However, Commodus was to inherit a realm that was stable and secure after the achievements of his father, Marcus Aurelius.
Life of Commodus
Commodus was born on August 31, 161 AD. His father was Emperor Marcus Aurelius, who was revered by his subjects and who was also a great philosopher. The young Commodus received an excellent education, based on Stoic principles and from an early age accompanied his father on his campaigns in the Marcomanni Wars. In 177 AD, he was made the joint ruler of the Empire with his father.
In 180 AD, Marcus Aurelius died while on campaign beyond the Danube. Commodus became sole and absolute ruler of the Roman world. He immediately paid a donative to the legions. To fund this, he devalued the coinage for the first time since the reign of Nero. Commodus continued the campaign against the German tribes on the Danube but soon entered into negotiations with the enemy. He signed a peace treaty that ended the Macromannic Wars, that had lasted almost 18 years.
Commodus returned to Rome, and after securing the loyalty of the Senate, he held a spectacular Triumph. It soon became apparent that Commodus did not have any interest in ruling and the day-to-day administration and soon he invested a favorite freedman with the management of the Imperial government. Commodus lived a very lavish existence, and he provided many mass entertainments and regularly gave the army donatives.
The elites quickly became dissatisfied with his rule and arbitrary tax demands on the senatorial elite. Commodus used the taxes to fund his extravagant spending. In 182 AD, a conspiracy organized by Commodus' sister was discovered. Not only was she executed, but the people who assisted her. After this Commodus became increasingly brutal and unpredictable and his mental health deteriorated. However, opposition was growing. He then had his chief minister executed to appease the legions. 
Later Commodus came under the complete influence of his mistress and her circle. Commodus increasingly became deranged. He was obsessed with gladiatorial games and even took part in combat. His participation was scandalous for an Emperor. He fought and killed many gladiators, whom he only armed with wooden weapons. Commodus later participated in the Games and killed many wild and exotic animals and numerous gladiators. In one inscription, Commodus claimed to have killed hundreds of men in the arena. 
The ruler of Rome began to portray himself as the demi-god Heracles. This was more than just propaganda; he appeared to believe that he was a manifestation of the legendary hero. It is claimed by Roman authors that he believed that he was the God when he was killing lions with a bow and arrow in the arena, just as the demigod had in the myths. Some believe that his identification with the divinity was a strategy to strengthen his rule and secure the population's obedience.
No one in the elite and his government was safe. By 190, Commodus was completely paranoid. The Emperor personally killed citizens who refused to attend the Games.  On December 31, 192, the Prefect of Rome and some members of the Emperor’s inner circle had him strangled while he was bathing with the champion wrestler, Narcissus. The Senate immediately proclaimed Pertinax, the city prefect, one of Commodus's assassins, the new Emperor.  His death marked the end of the Nerva-Antonine dynasty.
Ending the Macromannic War
By 180 AD, the legions had been at war for eighteen years. It seemed that Rome was on the verge of a complete victory and was about to annex the Marcomanni and their allies' territory, the Qadi. Marcus Aurelius hoped that one more campaign season would result in a complete and total victory and expand his Rome into central Europe. His death changed all of this. Soon after his death, Commodus ended the war, and many ancient writers regretted this, and later historians echoed this until recent times.
The wars of Marcus Aurelius on the Danube had strained the resources of the provinces, and the German tribes during their raids had devastated large parts of the Balkans and even Northern Italy. Many contemporary historians believe that Commodus was correct to end the wars. Rome was exhausted by war and also by the plague. The teenage ruler of much of the known world, possibly guided by his father’s advisors, ended the conflict.
If he had continued his father's policy and was successful in the wars, it is highly likely that the conquered lands would have only been a drain on the Imperial treasury and would have been impossible to defend. Commodus by ending the war, on favorable terms, had probably taken the correct strategic decision. He can be seen as continuing the policy of Augustus, which warned against further expansion.
Marcus Aurelius and his predecessors had cultivated a good relationship with the Senate. They had secured the cooperation and support of the senatorial elite. Commodus rejected his predecessors’ policy, and after been proclaimed Emperor, he side-lined the senators He taxed them very heavily, and this was regarded by many as an attempt to impoverish and weaken the senatorial elite, permanently. As his paranoia grew, he ordered the death of senators without even a treason trial.
This had not been done since the reign of Caligula. The successor of the great Stoic Emperor who esteemed the Senate treated the institution with contempt. Commodus wanted to be an absolute ruler, and this was to set the trend in Roman government. His treatment of the traditional elite set a precedent and his successors, beginning with Septimius Severus began to exclude the Roman nobility from government and did not even pretend to show them any deference. Commodus reign can be seen as initiating a new and more absolute concept of Imperial power and growing authoritarianism in the Imperial domains' government. 
Except for a few years in the reigns of Nero and Caligula, the Imperial territories had been ruled relatively well for two centuries. Successive Imperial governments had been fiscally prudent and had been careful to control spending. However, Commodus began to spend extravagantly, and this resulted in a financial crisis. Commodus's spending led to runaway inflation and inflation became a critical problem in the years ahead for the Roman Empire.
The last member of the Nerva-Antonine dynasty was a very insecure ruler, despite his claims to be Hercules' reincarnation.  He was very aware that he needed the legions' support. For the first time in decades, the Emperor sought to cultivate the army, to shore up his hold on power. It was another dangerous precedent, and it was one that was followed by his immediate successors. The result of which was that the legions, began to become increasingly powerful and this destabilized the Empire over the long term.
Civil War and the Year of the Five Emperors
Commodus's assassination was the first time in almost a hundred years that a reigning ruler of the Roman world was murdered. Furthermore, the last of his dynasty left no heir and abandoned adopting a son as his heir. The adoption of heirs had prevented any succession problems for many decades. The death of the son of Marcus and his misgovernment meant that after his death that there was a great deal of instability.
In 193 AD, five different men held the title of Emperor. The Praetorian Guard assassinated Petrinax when he tried to reform the Imperial Bodyguard. His successor bought the Imperial throne before Septimius Severus overthrew him.  Severus fought two bitter civil wars to secure his claim to the throne. Commodus's disastrous reign led to the most protracted period of instability in the Imperial lands since 66 AD. However, Septimius Severus proved that he was an able leader and stabilized the chaotic Empire a year after Commodus's assassination. The reign of the man who was obsessed with the Games and gladiators was one that ended the political stability that Rome had enjoyed for almost a century. 
Commodus was arguably one of the most respected Roman rulers and one of its greatest philosophers. The son was completely unsuited by temperament and character to be the sole ruler of an extensive state. He revolted against his father's influence and his Stoic education and acted contrary to what had been expected of him. He also became increasingly mentally unstable sometime during his reign.
He ended the Marcomanni War which was arguably the right strategic decision. However, his extravagance led to the devaluation of the coinage and inflation. Commodus rule was bloody, and his executions of elite members were something that had not been seen in over a century. The ruler also had a new concept of the Emperor as an absolute ruler, unrestrained by the Senate or even the law. His government was chaotic, and this led to civil war and instability after his death. Commodus reign was undoubtedly the end of an era, after 100 years of good government, the Empire experienced over a decade of misrule. In the 3rd century, it became increasingly authoritarian and more unstable, and some of this can be attributed to the years of Commodus rule.
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- Gibbon, The Decline and Fall of the Roman Empire (Hamondsworth, Penguin, 1984), p 17
- Gilliam, J. F. "The Plague under Marcus Aurelius". American Journal of Philology 82.3 (July 1961), pp. 225–251
- Geoff W Adams, Emperor Commodus: gladiator, Hercules or a tyrant? (Boca Raton, Brown Walker Press, 2013), p 17
- Adams, p 119
- Herodian, History of the Roman Empire since the Death of Marcus Aurelius, Book I, Ch. 1
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- Adams, p 118.
- Herodian, chp. 1
- McLynn, Frank, Marcus Aurelius, Warrior, Philosopher, Emperor (Vintage Books, London, 2009), p 279
- Historia Augusta, Life of Commodus, 6, 1
- Gibbon; I chapter 8
- Speidel, Michael P. "Commodus the God-Emperor and the army." The Journal of Roman Studies 83 (1993): 109-114
- Historia Augusta, Life of Petrinax, 8 2
- Speidel, p 110