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 judges.
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 Great Jewish Revolt. 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.
Crisis of the Third Century
With the assassination of the Emperor Alexander Severus on 18 March 235, the Roman Empire sank into a 50-year period of civil war, now known as the Crisis of the Third Century. The rise of the bellicose Sasanian Empire in place of Parthia posed a major threat to Rome in the east, as demonstrated by Shapur I's capture of Emperor Valerian in 259. Valerian's eldest son and heir-apparent, Gallienus, succeeded him and took up the fight on the eastern frontier. Gallienus' son, Saloninus, and the Praetorian Prefect Silvanus were residing in Colonia Agrippina (modern Cologne) to solidify the loyalty of the local legions. Nevertheless, Marcus Cassianius Latinius Postumus – the local governor of the German provinces – rebelled; 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. However, in the reign of Claudius Gothicus (268 to 270), large expanses of the Gallic Empire were restored to Roman rule. At roughly the same time, 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.