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In the early 2nd century AD, the Roman Empire had the Danube River as its northern border, but a powerful Dacian Kingdom was situated in the area of modern-day Romania. Under King Decebalus, Dacia united various tribes and bolstered its military strength, leveraging the Carpathian Mountains' terrain and its rich gold and silver resources. The clash between Rome and Dacia had already intensified during Emperor Domitian's reign. In 85 AD, when Dacian forces invaded Roman Moesia, Rome counterattacked but suffered heavy losses early on, with their commander Cornelius Fuscus even falling in battle. Rome subsequently negotiated a temporary peace with Dacia. However, the situation changed when Trajan became emperor. Trajan aimed to eliminate the military threat from Dacia, stabilize the border, and secure Dacia's mineral resources. Ultimately, tensions between the two nations escalated back into war, and Trajan prepared for a large-scale expedition. This campaign would significantly alter Rome's northern policy and lead to a more aggressive approach towards Dacia.

Emperor Justinian's territorial expansion was the most ambitious endeavor of the 6th-century Eastern Roman Empire to reclaim the past glory of the Roman Empire. Thanks to the efforts of Belisarius and Narses, the Vandal Kingdom and the Ostrogothic Kingdom were overthrown, and North Africa and Italy were brought back under control, with some territories also secured in the southeastern Iberian Peninsula. As a result, the empire could connect various ports and islands in the Mediterranean, exerting strong influence in the West. However, the conquest process involved prolonged warfare, consuming vast amounts of human lives and financial resources. Italy, in particular, suffered greatly from the Gothic Wars, and even after its conquest, continuous costs were required for defense and administration. Simultaneously, wars with Persia and invasions in the Balkans strained the empire's military forces, dispersing them across multiple regions. The territorial recovery achieved during his reign demonstrated the Eastern Roman emperors' will to carry on the Roman tradition. His expansion also laid a crucial historical foundation for the Eastern Roman Empire's self-understanding as the successor to the Roman Empire.

By the time Justinian died in 565 AD, the Eastern Roman Empire's territory had significantly expanded westward compared to when he ascended the throne. The empire controlled not only the Balkans, Anatolia, Syria, and Egypt, but also most of North Africa, Italy, and parts of the southeastern Iberian Peninsula. The Mediterranean Sea once again became the core area of activity for the Eastern Roman Empire, and cities like Carthage, Ravenna, Rome, and Constantinople were connected by the empire's rule. This was a result quite close to Justinian's dream of restoring the Roman Empire's territory. However, maintaining the conquered regions required enormous military and administrative expenses. Italy, in particular, suffered severe damage from the wars, and threats from Persia in the east and various forces in the Balkans persisted. Therefore, subsequent emperors found it difficult to preserve all the territories expanded by Justinian. His territory was later reduced again, but the memory lingered. It left a historical significance as the last large-scale attempt to revive the ancient Mediterranean world of Rome.

While expanding territories in the West, the Eastern Roman Empire continuously faced threats from the East and North. In 540 AD, Khosrow I of the Sasanian dynasty invaded Eastern Roman territory, reigniting the war with Persia. Justinian had to dispatch troops to both the West and the East simultaneously, and the war repeatedly saw truces and restarts. The troops and expenses required for the Eastern front became a significant burden for maintaining the Western conquests. In the Balkans, Slavic groups and various nomadic forces also crossed the borders, forcing the emperor to repair fortifications and deploy troops along the Danube. Thus, although the empire's territory greatly expanded, its military was inevitably spread across multiple fronts. In Italy, in particular, resistance and chaos continued even after the conquest, and garrisons were needed in North Africa and Iberia. Justinian's territorial expansion was not merely about acquiring conquered lands but became a challenge of simultaneously defending and governing vast regions. This burden became increasingly apparent as the emperor's reign progressed into its later years.

Justinian also sought to reclaim Rome's former influence in the Iberian Peninsula. At the time, most of Hispania was under the rule of the Visigothic Kingdom, but a succession dispute arose. Justinian exploited this, dispatching the Eastern Roman army around 552 AD. The troops advanced along the southeastern coast. The Eastern Roman army secured several ports, including Carthago Nova (Carthagena) and Malaca (Malaga), and controlled parts of the south and southeast. An Eastern Roman administrative district called Spania was established here. However, the Eastern Roman rule did not extend to the entire Iberian Peninsula, and the Visigothic Kingdom continued to hold most of the interior. Nevertheless, securing the southern Iberian ports was significant in strengthening the Mediterranean transport network connecting North Africa and Italy. Under Justinian, the Eastern Roman Empire's territory now extended not only to Africa and Italy but also to parts of Iberia. However, maintaining such widely dispersed territories required considerable military strength. As the scale of territorial expansion grew, so did the costs of border defense and administration.

Justinian's conquest of the West did not stop at North Africa and Italy. The Eastern Roman army strengthened the network between Western territories by securing key Mediterranean islands and coastal areas. Sicily became a crucial hub connecting Italy and North Africa, and Sardinia and Corsica also fell under the empire's influence. These islands were not merely territories but maritime strongholds for transporting troops and supplies and for operating fleets. With the establishment of sea lanes connecting Carthage in North Africa and Ravenna in Italy, the Eastern Roman Empire was able to exert considerable influence in the Western Mediterranean once again. However, maintaining expanded territories required stationing garrisons, collecting taxes, and operating administrative organizations in each region. The prolonged wars devastated rural areas and cities, placing a significant burden on local populations. Thus, while Justinian's conquests visually represented the revival of the Roman Empire on the map, actual rule demanded substantial costs. Sicily, in particular, was utilized as a key supply base linking North Africa and Italy.

As the Gothic Wars dragged on, Justinian utilized not only Belisarius but also Narses as key commanders. After Belisarius returned to Constantinople in 540, the Ostrogothic forces regained strength. Totila became king, seizing most of Italy, and Rome was attacked again, suffering heavy damage. In 552, Justinian dispatched Narses to Italy with a large army. Narses defeated Totila's forces at the Battle of Taginae, and Totila was killed. Following this, the Ostrogothic resistance rapidly weakened, and the last king, Theia, was defeated at the Battle of Mons Lactarius in 553. Thus, the Ostrogothic Kingdom effectively collapsed, and most of Italy came back under Eastern Roman rule. However, the conquest caused significant destruction to cities and the countryside, leading to a decrease in population and productivity. Although Justinian recovered the old territories, he had to commit continuous troops and finances to maintain Italy. Therefore, the Gothic Wars resulted in both a military victory and immense costs.

After collapsing the Vandal Kingdom, Justinian launched a new expedition to reclaim Italy, the heartland of the former Western Roman Empire. At the time, Italy was ruled by the Ostrogothic Kingdom, which maintained its own political system centered around Ravenna. The Gothic War began in 535 when Belisarius attacked Sicily. After securing Sicily, the Eastern Roman forces advanced into mainland Italy, capturing Rome by 536. However, the Ostrogoths did not collapse easily. A prolonged siege ensued as the army led by Witiges surrounded Rome, and the Eastern Roman forces defended the city despite shortages of food and manpower. The war then expanded throughout Italy, with battles fought over key cities like Ravenna and Milan. Although the Ostrogothic Kingdom seemed to crumble when Belisarius captured Ravenna in 540, new resistance soon began. The conquest of Italy took much longer than expected and placed a significant financial burden on the empire.

The Vandal Kingdom in North Africa was a state founded by the Vandals in the early 5th century, which controlled the maritime power of the Western Mediterranean centered around Carthage. Justinian pursued its conquest by exploiting the internal divisions and succession issues within the Vandal Kingdom. In 533, when the Eastern Roman army led by Belisarius landed in North Africa, the Vandal King Gelimer gathered his forces to resist. However, after winning the Battle of Ad Decimum, the Eastern Roman army captured Carthage and subsequently defeated the Vandal forces in the Battle of Tricamarum. Gelimer fled but eventually surrendered, and the Vandal Kingdom was officially dissolved in 534. North Africa came back under the rule of the Eastern Roman Empire, and Carthage became an important administrative and military center. This victory was the first successful large-scale conquest in the West initiated by Justinian. Furthermore, by securing the granary and ports of North Africa, the Eastern Roman Empire gained a foundation for conducting military operations in the Western Mediterranean. The next target was Italy, the heartland of the Western Roman Empire.

Before Justinian could launch his western campaigns, he needed to stabilize the threats from the east. The Eastern Roman Empire had long been in competition with the Sasanian Empire over Mesopotamia and Armenia. Although wars continued into the early years of his reign, the two powers signed the so-called "Eternal Peace" treaty in 532. Justinian paid a significant sum to Persia, temporarily halting the war on the eastern frontier and allowing him to concentrate his military forces in the west. Around the same time, the Nika riots erupted in Constantinople, throwing the capital into chaos, but the emperor suppressed the rebellion and consolidated his rule. With the internal and eastern situations stabilized, Justinian prepared his expeditionary forces and fleet. The Vandal Kingdom in North Africa, in particular, was experiencing internal divisions, making it a suitable target for attack. Justinian seized this opportunity, entrusting the expedition to Belisarius, and in 533, the first phase of the Vandalic War to reclaim western territories began. This was a crucial preparation for his western ambitions.