The Geopolitics of the Roman Empire deals with the "inalienable relationship between geography and politics of the Roman Empire". Once the Roman Empire had reached its natural borders, the location of potential threats to the empire and Roman troop locations played a major role in the elevation of Roman Emperors. Access to the troops, their location were crucial to the empire's internal politics, civil wars, and the eventual Fall of the Western Roman Empire.
Political factors Roman politics When Octavian declared himself Augustus and became the first Roman emperor in 27 BC, it ended a six-decade period of great internal instability within Roman politics. Already the ascent of Julius Caesar and the first triumvirate had proven the loyalty of the Roman military to their individual commanders rather than to the polity.
The military commanders had also used conquest as a way to gain prestige, this can be seen in the first triumvirate; Pompey and his conquests in the East, Caesar in Gaul and Cassius's disastrous attempt to invade the Parthian Empire. The senate lost its own power and influence In the face of the personal military power of individuals and their popularity with the plebeian masses.
These trends continued into the second triumvirate, the political structure from which Octavian rose and then defeated to became Caesar. Both Octavian and his main rival in the second triumvirate, Mark Anthony had a loyal personal military. Mark Antony had also attempted a failed campaign into Parthia through Armenia.
Neighbouring polities On the creation of the republic in 27 BC the empire was yet to reach its height. The Romans had pacified Gaul 20 years before during Caesars conquest and genocide of its people. His expedition into Germania at around the same time also stopped German incursions across the Rhine.
The only significant external power to the Romans was the Parthian Empire to the East. The two polities had strained ties, with Cassius and Mark Antony both leading failed invasions.
Geographic factors Natural borders of the Roman Empire
The Empire established by Augustus stayed consistent in territory for most of its history, though grew to permanently add Britannia through the campaign of Claudius and Dacia through Trajan's campaigns. Augustus had previously wanted the empire to incorporate territory until the Elbe river but changed his plans after the Battle of the Teutoburg Forest. Trajan also conquered Mesopotamia in addition to Dacia, but the territory was abandoned by the next emperor, Hadrian, who felt the territory would be too difficult and expensive to maintain against the Parthian Empire. The Empire's boundaries, therefore, were Hadrian's Wall (sometimes the Antonine Wall), the Rhine and Main rivers in the Western Empire. The Danube River in the middle empire. The mountains of Armenia and deserts of Syria to the East and the Sahara Desert and Atlas Mountains in Africa.
However, after the Battle of Adrianople and Theodosius's subsequent peace deal with the Goths the Rhine and Danube frontiers became incredibly porous. As per the peace deal, the victorious Goths retained their rights to pledge allegiance to their own King and retain their weapons, becoming in effect a separate state within the borders of the Roman Empire. Finding it harder to penetrate into the Eastern Roman Empire past the Bosporus, the Goths turned west. The Vandals, Franks and Alans would soon join the Goths in settling in Western Roman lands, while retaining their tribal allegiances and heritage.
Fracturing of the empire
The Empire fractured on many occasions, the first and most noteworthy being after the death of emperor Valerian, when the empire split into three parts; the Gallic Empire in the west, the Palmyrene Empire in the east and a rump middle empire led by Valerian's son, Gallienus. Conflict between the empires was initially averted as they were too busy dealing with external threats and a plague to fight amongst each other. However, the fracturing of the empire forced a reorganisation which allowed each part to hold back whatever threat they faced. The Gallic Empire stopped Frankish invasions, the rump Roman empire defeated an Alemanni force invading Italia, and Palmyra in the east under the stewardship of Odaenathus successfully fought off the Sassanid Empire. Modern historians credit this fracturing of the Empire as the reason that the Roman Empire endured under a period of intense duress, postulating that the three empires did a much better job than one Empire headed by a single emperor would have been able to do.
Indeed, the size of the empire and the inability for a single emperor to be able to effectively address barbarian invasions as well as internal threats of usurpation is one of the reasons Diocletian created the Tetrachy and the empire eventually split into its Eastern and Western halves.
The empire would continue to fracture during 407 AD, dissatisfied troops in Britannia hailed Constantine III as Emperor of the west. He then reached Gaul where he campaigned against a group of Vandals, Alans and Suebi who had crossed the Rhine.
Location of Roman capitals
During the Crisis of the Third Century, the presence of the emperor was required near the front to both subdue any barbarian invasion and to deter any possible usurpation or dissatisfaction from the troops. While the court moved with the emperor, during the height of the crisis the de facto capital moved to Mediolanum, present day Milan. The city rose as due to its proximity to the frontiers of both the Rhine and Danube rivers, where the Alemanni and Goths tribes would stage invasions of Roman lands. Mediolanum was the a large hub city within a significant road network including the Via Helvetica into Switzerland and France, Via Claudia Augusta into Austria and Southern Germany, as well as close proximity to the Via Germanica towards the Rhine. As such Mediolanum could facilitate rapid military and administrative movements across the empire. An elite cavalry force (Comitatenses) founded by emperor Gallienus was based in Mediolanum and could quickly be mobilised to counter barbarian incursions.
When Diocletian became emperor and formed the Tetrachy, the four cities of Nicomedia, Mediolanum, Sirmium and Augustus Treverorum became capitals of the Roman Empire. The reasons for choosing each capital are given below:
Augustus Treverorum, present day Trier in Germany, was chosen due to its proximity to the militarily active Rhine frontier and being a large city in the region with the infrastructure required to support the legionaries of the area. It was located on Via Agrippa, which lead to the wealthy provinces in the South of France and close to the Via Germanica and Via Aquitania which lead towards the Atlantic coast of France.
Sirmium, in present-day Serbia, was chosen for its proximity to the Danube frontier, where there were incursions from Gothic tribes. It lay on a road network that included the Via Aurelia which to the Adriatic sea, Via Gemina and Via Militaris which ran through the spine of the Balkans to the Aegean Sea and Anatolia.
Nicomedia was chosen by Diocletian as his capital due to its more central location between Europe and Asia. Once again, it was close to several strategic roads that ran between the Balkans and Anatolia and into Syria. Though in Asia, Nicomedia was close to Europe. During Diocleatian's reign it became the Empire's most populated city.
When Constantine I became sole Emperor he built a new capital at Constantinople because of its site with easy access to the Danube and Euphrates frontiers able to meet the European barbarian and Sassanid threat respectively as well as its more defensible location than Nicomedia, surrounded as it was with water on three sides.
After the battle of Adrianople, the Rhine and Danube frontiers became incredibly porous and an independent horde of Goths were loose within the borders of the empire. Alaric I's siege of Milan in 403 was enough to persuade emperor Honorius to move the capital to a less exposed location. Ravenna became capital of the Western Empire as it was surrounded by marshes on all sides and harder to lay siege to. The city also had access to the Adriatic Sea at a time when Western Roman Empire's greatest threat, the Visigoths had not managed to attain maritime supremacy.
External threats
Barbarian tribes living across the Rhine and Danube Rivers would often enter Roman territory for raids, attracted by the idea of plunder. This naturally led to the stationing of large numbers of troops on these frontiers to act both as defense and a deterrent. Similarly, the Parthian Empire to the east, later replaced by the Sassanids posed a large threat to Roman Syria and Egypt. Egypt was of particular importance to the empire due to its grain supply, so the Romans also left a large garrison in the east to counter this threat. The island of Britannia also had a large troop garrison to ensure the pacification of the many tribes inside and outside Roman territory.
Rivalries with Persia Armenia Geopolitically, Armenia was significant for the Roman Empire and the Persian Parthian and Sassanid Empires as it functioned as a crucial buffer zone. Control of Armenia would allow both parties to secure their borders while being able to project power into crucial provinces of their rivals. For the Romans, Armenia protected Persian incursions into the interior of Anatolia and Syria, both rich and important provinces for the empire. Armenia also sat on trade routes between Central Asia and Europe.
Both Romans and Persians tried to influence Armenia through the co-option of their royal family through marriage, as well through the support of client kings during periods of civil war, such as the Roman support of Tiridates I, which eventually lead to the Roman-Parthian War of 58-63 AD. Under Diocletion the Romans supported and installed Tiridates III as the King of Armenia, this again lead to war Roman–Sasanian War of 296–299 AD. At the end of this war, under the Peace of Nisibis the Romans also gained the right to appoint the Kings of Iberia, north of Armenia.
During the reign of Tiridates III, Christianity was proclaimed as the state religion, aligning the country more closely to Rome, especially after Constantine proclaimed the same for the Roman Empire.
Arab Kingdoms
Another buffer zone existed further south, around the area of present-day Jordan. Here Arab peoples lived on trade routes that stretched between Yemen and Syria. These routes were part of the ancient Indian Ocean trade network and transported valuable goods like pepper (which the Romans highly prized), incense, and silk from Asia to the Mediterranean.
For the Romans, control of these trade routes were important as these goods were prized in the empire. Both empires forged alliances with local Arab tribes and kingdoms by promising economic incentives and often stationed troops in these regions. This was to ensure that the client states remained aligned with their suzerain's respective interests.
Until its annexation by Rome in 106 AD, the Nabatean Kingdom, with its capital at Petra, was a key Roman client state. The Nabateans controlled crucial trade routes and provided a buffer against desert tribes and rival powers.
From around 200 AD the Ghassanids were a powerful Arab Christian tribe which allied with the Roman Empire and later Eastern Roman Empire. They served as a buffer against the Persian-aligned Lakhmids and helped defend the frontiers of the Roman Palestine and especially the incredibly wealthy and agriculturally productive province of Egypt from Persia.
Internal threats