Between 27 BC and 14 AD, Octavian became absolute ruler of the Roman Empire after the Roman Senate bestowed on him the title of Augustus. During his reign, longest of all the Caesars, Rome's domain reached the Danube. In response to new strategic and commercial needs, Augustus modernized the great system of highways between Italy and the central Danube area. As part of the new administrative regions of Venetia and Histria with Aquileia as their capital (formerly a mere strategic military base for the expansion of the Roman North-Eastern territories), today's Friuli became a vital part of Augustus' new communication system and administrative plan. By officially recognizing Aquileia's supremacy over the territory and cities situated between the Po Valley and the Istrian peninusula, Augustus hastened its development, eventually turning it into one of the most important cities of the Empire.
Situated far from the Empire's border marked by the Danube, protected by the Rhaetia, Noricum and Pannonia provinces, and favoured by their prestige and trade vitality, both Aquileia and Friuli enjoyed a particularly advantageous position within the universal Pax Romana Augusta. For the first three-hundred years of the Imperial Age (from the death of Augustus to Diocletian's retirement in 305 AD) Aquileia lived and prospered in the shadow of Rome, acting as the social and economic centre of its region and concentrating on peaceful activities. The increase in commercial traffic between the Mediterranean Sea and the Danube River, made this fluvial port city both powerful and splendid. It is estimated that Aquileia had more than 100,000 inhabitants, made up of descendents of the first colonists, as well as families of Celts, Histri, Veneti, Orientals from Asia Minor, Jews and Africans. According to the historian Decimius Magnus Ausonius (ca. 310-395 AD) Aquileia ranked fourth in importance among the Italian cities, the first three being Rome, Milan and Capua.
With the ascent to power of Constantine in 312 AD and his famous edict of 313 AD, which proclaimed a policy of of toleration towards Christianity, started a period of massive conversion of the city's population to the Christian faith. An important part of the active participation and writings of the Aquileian churchmen and scholars was directed towards overcoming the pagan mentality and old cults which still existed among the people. The construction of the so called post-Theodosius Basilica with a surface of some 2166 square metres and the nearby monastery evidenced the religious zeal in spreading the gospel among the local people.
With the death of Theodosius in 395 AD the Empire was yet again divided, this time between his two sons, who, because of their youth, were placed under the tutelage of the Roman general Stilicho, son of a Vandal captain. His policy of imperial toleration of all ethnic groups within the Empire, influenced by his fondeness of 'barbarians', encouraged the devastating phenomenon of invasions and failed miserably. The repeated barbarian invasions hastened the breakdown of the old institutions and imperial structure and deepened the rift between the Western and the Eastern Empire. Various barbaric tribes, pushed westward by the great migration of the Huns, a Mongol tribe, descending from the steppes of Central Asia, had already crossed the eastern borders of the Empire and settled throughout the Danube region setting up independent kingdoms of varying character (some largely Roman in culture and language, others almost wholly 'barbarian').
From the beginning of 400 AD, all the 'barbarians' who invaded Italy passed through the eastern alpine gate. Aquileia and Friuli stood right behind it and represented, of necessity, the first target for the aggressors. In 452 AD, Aquileia tried to prevent the passage of Attila and his Huns. The siege was long and exhausting. Legend has it that at one point the Huns had decided to abandon the siege, but the sight of a flock of storks abandoning their nests with the fledglings, revealed that the besiged were at the end of their food supplies. Finally the city surrendered and the assailants began killing and sacking, ultimately setting the city on fire. The news of the fall of Aquileia spread throughout Italy arousing fear and horror, becoming almost the symbol of the fate which hung over the Empire less than half a century later.