In modern historiography, the Western Roman Empire was the Roman Empire's western provinces collectively during any period in which they were administered separately from the eastern provinces by a separate, independent imperial court. Particularly from AD 395 to 476, there were separate, coequal courts dividing the governance of the empire into western and eastern provinces with a distinct imperial succession in the separate courts. The terms Western Roman Empire and Eastern Roman Empire were coined in modern times to describe political entities that were de facto independent; contemporary Romans did not consider the Empire to have been split into two empires but viewed it as a single polity governed by two imperial courts for administrative purposes. The Western Empire collapsed in 476, and the Western imperial court in Ravenna disappeared by 554, at the end of Justinian's Gothic War, marking one of the most transformative political changes during the era of late antiquity.
Although there were periods with more than one emperor ruling jointly before, the view that it was impossible for a single emperor to govern the entire Empire was institutionalized by emperor Diocletian following the disastrous civil wars and disintegrations of the Crisis of the Third Century. In 286 he introduced the system of the Tetrarchy, with two senior emperors titled Augustus, one in the East and one in the West, each with an appointed subordinate and heir titled Caesar. Though the tetrarchic system would collapse in years, the east–west administrative division endured in various forms. As such, the unofficial Western Roman Empire would exist intermittently between the 3rd and 5th centuries. Some emperors, such as Constantine I and Theodosius I, governed, if briefly, as the sole Augustus across the Roman Empire. On the death of Theodosius in 395, the empire was divided between his two infant sons, with Honorius as his successor in the West governing briefly from Mediolanum then from Ravenna, and Arcadius as his successor in the East governing from Constantinople.
In 476, the Roman army in the West was defeated in the Battle of Ravenna by Odoacer and his Germanic foederati. Odoacer forced Romulus Augustulus to abdicate and became the first King of Italy. That event was popularised by the 18th-century British historian Edward Gibbon as a demarcating the fall of the Western Roman Empire and is sometimes used to mark the transition from Antiquity to the Middle Ages. In 480, Western emperor Julius Nepos was assassinated and Eastern emperor Zeno dissolved the Western court and proclaimed himself the sole emperor of the Roman Empire. Odoacer's Italy and other barbarian kingdoms, many of them representing former Western Roman allies that had been granted lands in return for military assistance, would maintain a pretense of Roman continuity through the continued use of the old Roman administrative systems and nominal subservience to the Eastern Roman court.
In the 6th century, Emperor Justinian I re-imposed direct rule on large parts of the former Western Roman Empire, including the prosperous regions of North Africa, Italy and parts of Hispania. But political instability in the Eastern heartlands, foreign invasions, plague, and religious differences, made control of these territories difficult and they were gradually lost forever. Although the Eastern Empire retained territories in southern Italy until the 11th century, the Empire's influence over Western Europe had diminished significantly. The papal coronation of the Frankish king Charlemagne as Roman Emperor in 800 marked a new imperial line that would evolve into the Holy Roman Empire. It was a revival of the imperial title in Western Europe but not an extension of Roman traditions or institutions. The Great Schism of 1054 between the churches of Rome and Constantinople further diminished any authority the emperor in Constantinople could hope to exert in the West.
Background
As the Roman Republic expanded, it reached a point where the central government in Rome could not effectively rule the distant provinces. Communications and transportation were especially problematic given the vast extent of the Empire. News of invasion, revolt, natural disasters, or epidemic outbreak was carried by ship or mounted postal service, often requiring much time to reach Rome and for Rome's orders to be returned and acted upon. Therefore, provincial governors had de facto autonomy in the name of the Roman Republic. Governors had several duties, including the command of armies, handling the taxes of the province and serving as the province's chief houses.
Prior to the establishment of the Empire, the territories of the Roman Republic had been divided in 43 BC among the members of the Second Triumvirate: Mark Antony, Octavian and Marcus Aemilius Lepidus. Antony received the provinces in the East: Achaea, Macedonia and Epirus (roughly modern Greece, Albania and the coast of Croatia), Bithynia, Pontus and Asia (roughly modern Turkey), Syria, Cyprus, and Cyrenaica. These lands had previously been conquered by Alexander the Great; thus, much of the aristocracy was of Greek origin. The whole region, especially the major cities, had been largely assimilated into Greek culture, Greek often serving as the lingua franca.
Octavian obtained the Roman provinces of the West: Italia (modern Italy), Gaul (modern France), Gallia Belgica (parts of modern Belgium, the Netherlands and Luxembourg), and Hispania (modern Spain and Portugal). These lands also included Greek and Carthaginian colonies in the coastal areas, though Celtic tribes such as Gauls and Celtiberians were culturally dominant. Lepidus received the minor province of Africa (roughly modern Tunisia). Octavian soon took Africa from Lepidus, while adding Sicilia (modern Sicily) to his holdings.
Upon the defeat of Mark Antony, a victorious Octavian controlled a united Roman Empire. The Empire featured many distinct cultures; all experienced a gradual Romanization. While the predominantly Greek culture of the East and the predominantly Latin culture of the West functioned effectively as an integrated whole, political and military developments would ultimately realign the Empire along those cultural and linguistic lines. More often than not, Greek and Latin practices (and to some extent the languages themselves) would be combined in fields such as history (e.g., those by Cato the Elder), philosophy and rhetoric.
Rebellions and political developments
Minor rebellions and uprisings were fairly common events throughout the Empire. Conquered tribes or oppressed cities would revolt and the legions would be detached to crush the rebellion. While this process was simple in peacetime, it could be considerably more complicated in wartime. In a full-blown military campaign, the legions were far more numerous – as, for example, those led by Vespasian in the First Jewish–Roman War. To ensure a commander's loyalty, a pragmatic emperor might hold some members of the general's family hostage. To this end, Nero effectively held Domitian and Quintus Petillius Cerialis, Governor of Ostia, who were respectively the younger son and brother-in-law of Vespasian. Nero's rule was ended by a revolt of the Praetorian Guard, who had been bribed in the name of Galba. The Praetorian Guard, a figurative "sword of Damocles", was often perceived as being of dubious loyalty, primarily due its role in court intrigues and in overthrowing several emperors, including Pertinax and Aurelian. Following their example, the legions at the borders increasingly participated in civil wars. For instance, legions stationed in Egypt and the eastern provinces would see significant participation in the civil war of 218 between Emperor Macrinus and Elagabalus.
As the Empire expanded, two key frontiers revealed themselves. In the West, behind the rivers Rhine and Danube, Germanic tribes were an important enemy. Augustus, the first emperor, had tried to conquer them but had pulled back after the disastrous Battle of the Teutoburg Forest. Whilst the Germanic tribes were formidable foes, the Parthian Empire in the East presented the greatest threat to the Empire. The Parthians were too remote and powerful to be conquered and there was a constant Parthian threat of invasion. The Parthians repelled several Roman invasions, and even after successful wars of conquest, such as those implemented by Trajan or Septimius Severus, the conquered territories were forsaken in attempts to ensure a lasting peace with the Parthians. The Parthian Empire would be succeeded by the Sasanian Empire, which continued hostilities with the Roman Empire.
Controlling the western border of Rome was reasonably easy because it was relatively close to Rome itself and also because of the disunity among the Germans. However, controlling both frontiers simultaneously during wartime was difficult. If the emperor was near the border in the East, the chances were high that an ambitious general would rebel in the West and vice versa. This wartime opportunism plagued many ruling emperors and indeed paved the road to power for several future emperors. By the time of the Crisis of the Third Century, usurpation became a common method of succession: Philip the Arab, Trebonianus Gallus and Aemilianus were all usurping generals-turned-emperors whose rule would end with usurpation by another powerful general.
The idea of co-emperorship was first tested by Marcus Aurelius (r. 161–180), who decided to rule alongside his adoptive brother Lucius Verus. There was, however, much precedent. The consulate of the Republic was a twin magistracy, and earlier emperors had often had a subordinate lieutenant with many imperial offices. Many emperors had planned a joint succession in the past—Augustus planned to leave Gaius and Lucius Caesar as joint emperors on his death; Tiberius wished to have Caligula and Tiberius Gemellus do so as well; as Claudius with Nero and Britannicus. All of these arrangements had ended in failure, either through premature death (Gaius and Lucius) or murder (Gemellus and Britannicus). Marcus Aurelius ruled mostly from the West, while Verus spent most of his reign campaigning in the East, against Parthia. Verus accompanied Marcus at the start of the Marcomannic Wars, but died shortly after. Decades later, Septimius Severus (r. 193–211) appointed his sons Geta and Caracalla as joint heirs. However, Caracalla murdered his brother shortly after succeeding to the throne.
Crisis of the Third Century
With the assassination of the emperor Alexander Severus in March 235 by his own troops, the Roman Empire sank into a 50-year period of civil war now known as the Crisis of the Third Century. During this period, the Empire saw the combined pressures of barbarian invasions and migrations into Roman territory, civil wars, peasant rebellions and political instability, with multiple usurpers competing for power. The idea of dividing the empire was first developed in this time; Valerian and his son Gallienus divided the empire between them, the former ruling from the East and the latter from the West. However, Valerian was captured by the Sassanids at the Battle of Edessa, leaving Gallienus as sole emperor.
Saloninus, Gallienus' infant son, and the praetorian prefect Silvanus resided in Colonia Agrippina (modern Cologne) to solidify the loyalty of the local legions. Nevertheless, Postumus – the local governor of the German provinces – rebelled, and his assault on Colonia Agrippina resulted in the deaths of Saloninus and the prefect. In the confusion that followed, an independent state known in modern historiography as the Gallic Empire emerged.
Its capital was Augusta Treverorum (modern Trier), and it quickly expanded its control over the German and Gaulish provinces, all of Hispania and Britannia. It had its own senate, and a partial list of its consuls still survives. It maintained Roman religion, language, and culture, and was far more concerned with fighting the Germanic tribes, fending off Germanic incursions and restoring the security the Gallic provinces had enjoyed in the past, than in challenging the Roman central government.
In the reign of Claudius Gothicus (268–270), large expanses of the Gallic Empire were restored to Roman rule. At roughly the same time, however, several eastern provinces seceded to form the Palmyrene Empire, under the rule of Queen Zenobia.
In 272, Emperor Aurelian finally managed to reclaim Palmyra and its territory for the empire. With the East secure, his attention turned to the West, invading the Gallic Empire a year later. Aurelian decisively defeated Tetricus I in the Battle of Châlons, and soon captured Tetricus and his son Tetricus II. Both Zenobia and the Tetrici were pardoned, although they were first paraded in a triumph.
Tetrarchy
Diocletian divided the Roman Empire when, in 286, he elevated Maximian to the rank of Augustus (emperor) and gave him control of the Western Empire, while he continued to rule the East. In 293, Galerius and Constantius Chlorus were appointed as their subordinate (caesars), as a way to avoid the civil unrest that had marked the 3rd century. This system effectively divided the Empire into four major regions, the First Tetrarchy: in the West, Maximian made Mediolanum (now Milan) his capital, and Constantius made Trier his. In the East, Galerius made his capital Sirmium and Diocletian made Nicomedia his. On 1 May 305, Diocletian and Maximian abdicated, replaced by Galerius and Constantius, who, in turn, appointed Maximinus II and Valerius Severus, respectively, as their caesars, thus creating the Second Tetrarchy.
The Tetrarchy collapsed after the unexpected death of Constantius in 306.
His son, Constantine, was declared Western emperor by the British legions, but several other claimants arose and attempted to seize the Western Empire. In 308, Galerius revived the Tetrarchy by dividing the Western Empire between Constantine and Licinius. However, Constantine was more interested in conquering the whole empire so, by 314, began to compete against Licinius, finally defeating him in 324 at the Battle of Chrysopolis. After Constantine unified the empire, he refounded the city of Byzantium in modern-day Turkey as Nova Roma ("New Rome"), later called Constantinople, and made it the capital of the Roman Empire. Although the Tetrarchy was ended, the concept of physically dividing the Roman Empire into East and a West continued, as happened after the deaths of Constantine and Theodosius I.
Further divisions
The Roman Empire was under the rule of a single emperor, but, with the death of Constantine in 337, the empire was partitioned between his surviving male heirs. Constantius, his third son and the second by his wife Fausta (Maximian's daughter) received the eastern provinces, including Constantinople, Thrace, Asia Minor, Syria, Egypt, and Cyrenaica; Constantine II received Britannia, Gaul, Hispania, and Mauretania; and Constans, initially under the supervision of Constantine II, received Italy, Africa, Illyricum, Pannonia, Macedonia, and Achaea. The provinces of Thrace, Achaea and Macedonia were shortly controlled by Dalmatius, nephew of Constantine I and a caesar, not an Augustus, until his murder by his own soldiers in 337. The West was unified in 340 under Constans, who was assassinated in 350 under the order of the usurper Magnentius. After Magnentius lost the Battle of Mursa Major and committed suicide, a complete reunification of the whole Empire occurred under Constantius in 353.