Tetrarchy Map and Rule in the Roman Empire


Diocletian and the Roman Tetrarchy

Diocletian’s Tetrarchy Rule in the Roman Empire with Maps

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’s era (96-192) and it continued during the Severan Dynasty’s era (193-235) despite ups and downs.

After Severus Alexander, the last member of the Severan Dynasty, was executed by his own army in 235 A.D., a long (235-285) “Imperial Crisis” that stormed the empire broke out. 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 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’ son. 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 abduction of the throne. Diocletian was a very unusual man and he was the first emperor in the Roman Empire that would abduct his throne willingly. After his abduction of the throne, he spent his retirement years by doing farming at Split in Croatia.

Tetrarchy Rule in the Roman Empire

Diocletian probably had tendencies to share the power even before he claimed the throne. Five Good Emperors period, the most prosperous era of the empire, came to an end with Marcus Aurelius’ suspicious death and his son Commodus’ (The terrible emperor in the movie Gladiator) ascendance of the throne. And a promising emperor named Probus that claimed the throne at the end of 50 years long Imperial Crisis period was killed by Carus’ soldiers.

Maximian Becomes “Augustus”

These coups that prevented the empire from being ruled consistently led to the belief among public that the new emperor won’t be long lived. 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 states, Maximian, the co-emperor, ruled the West.

During these years, the Roman Empire had big problems both in the East and the West. According to Diocletian, one ruler would be unable to solve all of these problems and one emperor wouldn’t be able to fight on two grounds at the same time. 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 rightful ruler of the Roman Empire. 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.

Two Caesars Appointed and Tetrarchy Era Begins

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 thought the co-emperor system wasn’t enough either and promoted two generals, Constantius Chlorus in the West and Galerius in the East, as Caesars. As a result, the hierarchical system came out as below:

Augustus (The Senior Emperors)

Maximian (the West) < > Diocletian (the East)

Caesar (The Junior Emperors)

Constantius (the West) < > Galerius (the East)

 Note: Diocletian was the veteran emperor and he was entitled to appoint others.

Tetrarchy Map of the Roman Empire

Roman Empire Tetrarchy Map

Diocletian, Maxiamian, Gallerius, Constantius

Constantius Takes Britain Back

Constantius (father of Constantine the Great) was 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 his General Asclepiodotus simultaneously. Therefore, the hardest operation (the recapturing of Britain) of its time was carried out successfully.

Galerius’ Fight against the Persians

An emperor named Narseh became the ruler of Sasanians, also known as the 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. Even the Harem tent of Narseh’s wives and kids were seized. Therefore, Valerian, who died during his captivity, was avenged.

The Tetrarchy in the Roman Empire on a Map

Tetrarchy Map

Tetrarchy Rule

Abdication of the Emperor Diocletian

After 20 years’ of rule, Diocletian decided to abduct the throne. He wanted the Tetrarchy that he established to continue. Therefore, he encouraged Maximian to abduct his 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 also appointed as Caesar in the East.

Augustus (the Senior Emperors)

Valerius Severus (the West) < > Galerius (the East)

Caesar (the Junior Emperors)

Constantine (the West) < > Maximinus Daia (the East)

                Note: Galerius was the senior emperor. No appointment without his confirmation.

Rebellion against Severus and Maxentius’ Demand for Legitimacy

This was the new order, however, it didn’t last long. As “Augustus” Severus, who ruled Italy and nearby regions, crushed his subjects with heavy taxes, a huge rebellion broke out in Rome.

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 (the North Africa) (Note: General Pompeianus commanded the campaign) and he annexed it. However, the senior emperor Galerius at Nicomedia (modern day Izmit) 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 both the public didn’t like Severus and most of the soldiers 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.

Galerius’ March towards Italy

As Galerius received the news, he went mad. He gathered his army and launched his campaign on Rome. 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 fight 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.


Galerius leading the Roman Legions

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.

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.

Augustus (the Senior Emperors)

Licinius (the West) < > Galerius (the East)

Caesar (the Junior Emperors)

Constantine (the West) < > Maximinus Daia (the East)

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, the retired emperor caused trouble 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. The system he and Diocletian created was going to become history soon like he did.

The Death of Galerius and the End of Tetrarchy

Meanwhile, something unexpected happened and Emperor Galerius died. “Caesar” Maximinus Daia declared himself as Augustus in the East and seized the capital Nicomedia. On the other hand, Constantine would call himself “Augustus” for a long time back then. Therefore, the new order that was agreed in the meeting with Diocletian became obsolete in just a few years.

When all the kings from his father’s era (Maximian, Diocletian, and Galerius) died, Constantine started thinking that his time to become the ruler came.

Map of Tetrarchy after the death of Galerius

Battle of the Milvian Bridge

Constantine the Great and Tetrarchy

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 ¾ 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.

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.

They burnt down all the bridges to prevent Constantine from using them and there were only two bridges left remaining, which were almost ruined. As the bridge that stood on weak ropes collapsed with hundreds of soldiers on it, the other soldiers ran to a 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.

Constantine crush Maxentius

The Battle of the Milvian Bridge

Constantine against Maxentius

Licinius and Constantine Forms an Alliance

Constantine established a total rule over the Western Roman Empire 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 took control of the whole Middle East after the death of Galerius. Despite having a relatively large army, he was beaten by Licinius.

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 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.

The Final: Constantine and Licinius Fights

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 last battle fought.

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.

He left Rome that he didn’t really like and he decided to create a new capital in the East. Although his commanders expected him to declare Nicomedia as the new capital, he chose a brand new capital Byzantium.

The Byzantium, which Constantine rebuilt thoroughly, would form the core of the Byzantine Empire under the name Constantinople. Constantine’s adoption of Christianity and moving the capital to the east extended the empire’s life to another millennium under the name of the Eastern Roman Empire.

Roman Empire Tetrarchy Map by Serhat Engul


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