The Tetrarchy symbolizes a transitional period in the Roman Empire. The Tetrarchy (Rule of Four) was brought by Diocletian and put an end to ciwil wars in Rome for a while. However, this new system caused more wars afterwards.
It may be surprising to come across such a detailed article on Tetrarchy in an Istanbul blog. However, the road to the establishment of Constantinople and its declaration as the capital of Roman Empire began with the Tetrarchy.
In this article you can find maps depicting the Tetrarchy period. In addition, the Tetrarchy emperors and the ciwil wars that took place in this period are noted in detail. In order to explain this period, I would like to make a brief introduction to Roman history.
Table of Contents
Crisis of the Third Century
After Julius Caesar’s adopted son Octavius was enthroned as “Augustus”, the Roman Empire witnessed a long rising period. The prosperity of the empire peaked during Nerva-Antonine Dynasty (96-192) and it continued during the Severan Dynasty (193-235) despite ups and downs.
When Severus Alexander was executed by his own soldiers in 235, a period of turmoil called the Crisis of the Third Century began in the history of the Roman Empire. During this 50 years of long crisis period, emperors were assassinated one after another and they couldn’t rule the empire more than a couple of years.
The emperor who ended this chaotic era was Emperor Diocletian. Diocletian served as an officer in the cavalry army of Emperor Carus and rose rapidly due to his military success.
After Carus’ unexpected death, his son Carinus and Numerius seized the power. However, the prominent figures of the Roman Empire and Diocletian, the most popular general of the army, didn’t want Carus’ sons. Therefore, they settled accounts at the Battle of the Margus nearby Serbia. Carinus died in the battle and his brother Numerius was probably assassinated.
In the end, Diocletian claimed the throne as the Roman Emperor in 284. Diocletian’s 20 years reign began in the empire that would end by his voluntary abdication. Diocletian was a very unusual man and he was the first emperor in the Roman Empire that would abdicate the throne willingly. He spent his retirement years by doing farming at Split in Croatia.
The Roman Tetrarchy (Rule of Four)
Diocletian probably had tendencies to share the power even before he claimed the throne. Because he knew that one of the biggest problems in the Roman Empire was the succession system. Even after the most prosperous times, civil wars were inevitable when the emperors did not leave a decent heir.
Another problem was that Rome was too big to be ruled by a single person. Thus, Diocletian decided on the The Roman Tetrarchy (Rule of Four) system, which divided the empire between two senior rulers and included the junior emperors who would follow them.
The two senior emperors who ruled the empire during the Tetrarchy would be called “Augustus” and the two junior emperors under them would be called “Caesar”. This system caused great wars that would change Roman history. You can find all the details about this period in the following 15 titles.
1. Maximian Becomes “Augustus”
One of the reasons Diocletian wanted to introduce the Tetrarchy was to ensure continuity in administration. Because during the crisis of the 3rd century, the life of the emperors was very short due to coups. There was a perception among the people and the soldiers that the emperor who ascended the throne could not stay in power for more than a few years.
As Diocletian was aware of this perception, he was worried that the reforms he was going to initiate would be left unfinished. Therefore, his first action was to divide the rule into two. While he ruled the Eastern provinces, Maximian would become the second “Augustus” and rule the West.
During these years, the Roman Empire had big problems both in the East and the West. According to Diocletian, an emperor engaged in reforms could not simultaneously lead wars. In order to make quick and firm decisions, two rulers with absolute authority in both sides were necessary.
There was a fight with Persian Empire over Syria and Armenia in the East. On the other hand, there was a usurper emperor named Carausius in the Britain. He claimed himself as the “Emperor of the North”. The first thing Diocletian expected from his colleague Maximian was to take Britain back.
Although Carausius was seen as a usurper by the Romans, he took control of the legions in Britain and he was accepted by the public. Also, he managed the trade in Britain successfully. As he brought wealth and prosperity, he was supported both in England and nearby countries.
Under these circumstances, Maximian’s march to Britain failed. A storm destroyed the ships of the Roman Empire’s navy and the campaign (290) launched with limited force turned out to be a flop.
2. The Roman Tetrarchy Map
Diocletian knew the empire was in danger of being torn apart. As was the case in Britain and Egypt later, the commanders in the army started to declare themselves as Tyrants and steal lands from the Roman Empire.
Diocletian saw that the co-imperial system did not solve the problems either, and promoted two generals, Constantius Chlorus in the West and Galerius in the East, as Caesars. Now let’s take a look at the power sharing of the Tetrarchy (Rule of Four).
On The Roman Tetrarchy Map above you can see exactly how the emperors shared the lands. Maximian rules the West as “Augustus” and Diocletian rules the East as “Augustus” as senior emperors.
Constantius as “Caesar” rules in the West and Galerius as “Caesar” in the East as junior emperors. The Caesars’ task is to quickly move into troubled areas and lead wars. It is Diocletian who has absolute authority among all rulers and makes the final decisions regarding appointments.
What was expected of the newly appointed caesars was the rapid resolution of problems requiring military intervention. To this end, Constantius was to march on the usurper in Britain. Galerius, on the other hand, would resolve border conflicts with the Persians.
3. Constantius Recaptured Britain
Constantius Chlorus (father of Constantine the Great) had been a high ranking commander in Maximian’s army. He drew lessons from the failed campaign against Britain. Therefore, he applied different tactics this time. He took control of all the ports in Gaul and blocked all the trade routes to Britain, therefore hitting Britain where it hurts.
Until then, Carausius ruled Britain successfully but his enemies took advantage of the fact that the island was isolated and assassinated him. The leading figure of the plot was a bureaucrat named Allectus and he declared himself as the new Augustus. However, his rule turned out to be a short one.
Constantius charged against the enemy. He divided his army into two to puzzle the enemy and he took Allectus by surprise by marching towards the enemy with General Asclepiodotus simultaneously. Therefore, the hardest operation (the recapturing of Britain) of its time was carried out successfully.
Constantius had recaptured Britain, and from now on his name would be known as the “Conqueror of Britain”. Constantine the Great would benefit greatly from his father’s prestige in the future.
4. Galerius’ Fight against the Persians
An emperor named Narseh became the ruler of Sasanians (Second Persian Empire) and he wanted to follow the footsteps of his father Shapur because his father attacked Antioch, a land that belonged to the Roman Empire, and he took the Roman Emperor Valerian hostage. And Narseh, in the same fashion, wanted to seize Syria and the nearby lands.
Galerius was after avenging Valerian who died under captivity. Moreover, he wanted the end Persians’ struggle for power over Syria too. The first clash of the two emperors turned out to be a disaster for the Romans. Galerius was beaten at the Battle of Carrhae (modern day Harran) and they withdrew to Antioch. Diocletian got mad at the result and pressured his subordinate Galerius to finish the task he started.
The second war (the Battle of Satala) took place in Armenia. Galerius was stronger thanks to the reinforcements from the Balkans and he was ready for the battle and the land structure was suitable for the legions. On the other hand, the movement area of the Persian cavalry, which was a decisive factor on the outcome of the former battles, was limited this time.
The Romans were the clear winners of this battle thanks to the conditions that were in their favor. Galerius’ fight against the Persians ended in victory, and even the Harem tent of Narseh’s wives and kids were seized. Therefore, Valerian, who died during his captivity, was avenged.
5. Abdication of the Emperor Diocletian
After 20 years’ of rule, Diocletian decided to abdicate the throne. He wanted the Tetrarchy that he established to continue. Therefore, he encouraged Maximian to abdicate the throne simultaneously. Even though Maximian was not so willing to do so, he had to accept his fate.
Therefore, Constantius in the West and Galerius in the East were promoted to being “Augustus”. However, Constantius, “the Conqueror of Britain”, lost his life just after one year of his promotion. Following his death, his son, Constantine was declared as “Augustus” by the Western army.
However, Galerius didn’t accept this fait accompli. He appointed Severus, his old friend from the army, as Augustus in the West and the young Constantine was his subordinate as Caesar. The young emperor who would be called “Constantine the Great” in the future took this promotion as satisfactory “for now”. Also, Maximinus Daia was appointed as Caesar in the East.
In short, while the senior emperors were Galerius and Severus, the junior emperors were Constantine and Maximinus Daia.
6. Rebellion against Severus
This was the new order, however, it didn’t last long. Severus, who ruled Italy and the surrounding lands, oppressed the people with heavy taxes. For this reason, a great rebellion broke out in Rome against Severus.
The commanders of the army took side with the public and they declared Maxentius, son of the former “Augustus” Maximian, as the emperor. Maxentius was already expecting to be appointed at least as Caesar after his father’s abdication and it was time to take what was his.
Maxentius moved to Italy, where his father ruled before, and grew stronger there. He defeated the usurper Domitius Alexander who declared himself as the emperor of Carthage and he annexed it. However, the senior emperor Galerius at Nicomedia didn’t approve Maxentius’ rule and ordered Severus to take Rome back.
However, as Severus was coming closer to lay siege to Rome, his soldiers were reluctant to fight on his side, because most of the Severus’ soldiers had served Maximian who was the former Augustus. They didn’t want to fight against his son Maxentius.
As Severus felt helpless, he wanted to fall back to Ravenna and gather men. However, he was too weak even to accomplish it. He surrendered to Maximian in exchange for his life to be pardoned. However, Maximian and his son Maxentius didn’t keep their promise and they killed Severus.
7. Galerius’ March on Italy
As Galerius received the news, he went mad. Galerius decided to march on Italy with his army to restore order for the Tetrarchy. However, he realized quickly that it wasn’t easy to seize Italy. The cities blocked their way and his moves became restricted. He had to withdraw his army in order not to have the same fate as Severus.
Galerius decided to get the advice of Diocletian who was enjoying his retirement days. It was nobody but Diocletian whose words would be listened in the days that Tetrarchy was in crisis. After all, Diocletian was the founder of Tetrarchy system.
After Galerius’ unsuccessful Italy campaign, a struggle for power began between the former emperor Maximian and his son Maxentius. Maxentius was grateful to his father for helping him in his fight against Severus and Galerius. However, he didn’t want his father to overshadow his authority. As a result, Maximian was exiled from Italy as he did things against his son.
Maximian went to Galerius’ palace at Nicomedia and asked for refuge. Normally, Galerius would be expected to punish Maximian after what he did in Italy. However, as he didn’t want this incident to overshadow his upcoming meeting with Diocletian, he decided to bring Maximian with him to this meeting.
8. Tetrarchy Rule is Reorganized
The three emperors met after long years. Maximian begged Diocletian to come back. He wanted to rule as “Augustus” like before. However, Diocletian had no interest in going back to politics. And he warned Maximian not to meddle with Tetrarchy.
As a result of the meeting, the Tetrarchy rule was reorganized. According to the new system, Licinius became “Augustus” by Galerius’ recommendation. Meanwhile, Constantine and Maximinus Daia remained as Caesar. Constantine’s expectation to be appointed as Augustus didn’t come true and Maxentius was declared as usurper in Italy.
In short, while the senior emperors were Galerius and Licinius, the junior emperors were Constantine and Maximinus Daia.
9. Maximian Creates a New Problem
After being exiled from Italy by his son and not being welcomed well by Galerius in the East, Maximian decided to move into his son-in-law Constantine’s.
Constantine was married to the former Augustus Maximian’s sister Fausta. Therefore, he didn’t treat him badly despite his usurper son Maxentius. However, Maximian continued to create new problems here too. As he tried to provoke Constantine’s legions in Gaul against him, he was forced to commit suicide.
Maximian, who served as “Augustus” with Diocletian for 20 years and was also the co-founder of Tetrarchy system, became history. And soon the system they had established with Diocletian would also be history.
10. The Death of Galerius
Meanwhile, something unexpected happened and Emperor Galerius died. Galerius was the insurance of the Tetrarchy system and the death of Galerius plunged Rome into new crises.
“Caesar” Maximinus Daia declared himself as Augustus in the East and seized the capital Nicomedia. The other “Caesar” Constantine immediately declared himself Augustus as well. Therefore, the new order that was agreed in the meeting with Diocletian became obsolete in just a few years.
When all the rulers from his father’s era (Maximian, Diocletian, and Galerius) died, Constantine started thinking that his time to become the sole ruler came.
11. The Battle of the Milvian Bridge
Although I haven’t mentioned it so far, a new period that would change the fate of the Roman Empire began. Just in 20 years, the religion of the Roman Empire was going to change from Paganism to Christianity and the new capital was going to be Constantinople (Istanbul). The person who was going to make these changes was Constantine, the son of Constantius Chlorus.
Constantine’s steps to create a one-man rule started with beating his enemy Maxentius in the South. Constantine deployed most of his troops that he left in Britain, Gaul, and Spain and he began to march to Rome with his army of 30,000 soldiers.
He seized Torino and Milano with ease, but he faced a serious opposition at Verona. However, the way to Rome was open after the death of Pompeianus in the Battle of Verona, the most important man of Maxentius.
Although Maxentius decided to stay in defensive position till the last moment, he faced Constantine on the battleground by leaving Rome. The army of Maxentius was larger and he actually had a chance to beat Constantine.
However, the soldiers in Constantine’s army were strong soldiers who had great experience in years. Moreover, they were loyal to Constantine to the death. Maxentius’ soldiers, on the other hand, tended to panic if things went wrong, which was the case during the Battle of the Milvian Bridge.
Constantine’s troops destroyed Maxentius and despite being relatively fewer in numbers, they didn’t let the enemy surround them. Maxentius couldn’t take the pressure and he wanted to fall back to Tiber River and gather his power back. However, he couldn’t manage it.
Maxentius had actually fallen into his own trap. He had destroyed all the bridges so that Constantine could not cross, and there was only two bridges he could use to retreat. And they were almost a ruin, too. As the bridge that stood on weak ropes collapsed with hundreds of soldiers on it, the other soldiers ran to the stone bridge called the Milvian Bridge that was too narrow.
However, most of them were crushed and fell to the river. Maxentius was one of the people that fell into the river and he died by drowning. And Constantine was welcomed like a hero when he arrived Rome.
12. Licinius and Constantine
Constantine established a total rule over the Western Europe while Licinius was in power in the Balkans. These two rulers made a deal and the aim of the deal was to get rid of Maximinus Daia who controlled all the trade routes in the Middle East.
Maximinus Daia had taken control of the whole Middle East after the death of Galerius. Despite having a relatively large army, he was beaten by Licinius. Now only Licinius and Constantine remained of the Tetrarchy.
Meanwhile, Maximinus Daia was the only emperor who was still in favor of the polytheist Pagan religions and the Jupiter Cult. Following the victory of the Milvian Bridge, Constantine had declared the the Edict of Milan and he ended the mistreatment of Christians that lasted for 300 years. Licinius also appeared to be in favor of this decision.
13. Battle of Chrysopolis
Tetrarchy, which was founded by Diocletian, collapsed once and for all. As just before Tetrarchy was declared, there were two emperors on each side, which were Licinius in the East and Constantine in the West. However, although he wedded his daughter to Licinius and became relatives with him, Constantine had the desire to be the one and only emperor of the Roman Empire.
Licinius, on the other hand, was involved in an assassination attempt against Constantine and he basically put his head in a noose. Constantine won the battle against Licinius that took place in the Balkans (Adrianople) and a short period of peace phase began. However, the fierce treatment of Licinius towards the Christians caused a great reaction in the West.
Constantine wanted to end the co-ruling system permanently and by crossing the Dardanelles Strait (Hellespont) he crashed Licinius’ army that was in a defensive position on the Bosphorus. The army of Licinius fled to Chrysopolis (Uskudar) on the Asian Side and they were destroyed there at the Battle of Chrysopolis.
Licinius was taken as hostage and although he was forgiven by Constantine at first, he was executed later. According to some sources, the reason for the change of decision was his desire to gather an army and fight against Constantine.
Therefore, the last emperor from the Roman Tetrarchy Rule also became history and Constantine, who never lost a single battle on the battlefield till then, became the only ruler of the Roman Empire.
14. Foundation of Constantinople
Constantine disliked the old aristocracy of Rome. Moreover, Rome had already lost its function of being the center of the empire. During the Tetrarchy period, senior emperors such as Diocletian and Galerius lived in Nicomedia in the East.
Constantine decided to establish a new capital. While fighting Licinius, he realized the strategic importance of the Bosphorus and admired the natural protection Byzantium provided.
Byzantium was declared the new capital and was completely rebuilt in 6 years. The foundation of Constantinople was celebrated with ceremonies in 330. The most important buildings of the new capital were the Hippodrome of Constantinople, the Forum of Constantine and the Church of the Holy Apostles.
15. The Byzantine Empire
Constantine’s move of the Roman Empire’s power center to the east allowed the Roman cult to survive for another 1000 years. Combining the administrative, military and architectural foundations of Rome with the Ancient Greek culture, the Byzantine Empire lived until 1453.
Although the “Eastern Roman Empire” is referred to in modern history as the Byzantine Empire, they had always considered themselves the Romans and successors of Constantine the Great.
While writing this article, I benefited from Mike Duncan‘s The History of Rome podcast and John Julius Norwich‘s book called Byzantium. I used online encyclopedias like Wikipedia and Britannica to double check some of the information. I would also like to thank those who prepared the anonymous maps about the history of the Roman Empire.
Roman Tetrarchy Map and Civil Wars by Serhat Engul