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As Western Europe witnessed the formation of new kingdoms, the Eastern Roman Empire remained intact and experienced an economic revival that lasted into the early 7th century. There were fewer invasions of the eastern section of the empire; most occurred in the Balkans. Peace with Persia, the traditional enemy of Rome, lasted throughout most of the 5th century. The Eastern Empire was marked by closer relations between the political state and Christian Church, with doctrinal matters assuming an importance in eastern politics that they did not have in Western Europe. Legal developments included the codification of Roman law; the first effort—the Theodosian Code—was completed in 438. Under Emperor Justinian (r. 527–565), another compilation took place—the Corpus Juris Civilis. Justinian also oversaw the construction of the Hagia Sophia in Constantinople and the reconquest of North Africa from the Vandals and Italy from the Ostrogoths, under Belisarius (d. 565). The conquest of Italy was not complete, as a deadly outbreak of plague in 542 led to the rest of Justinian's reign concentrating on defensive measures rather than further conquests. At the emperor's death, the Byzantines had control of most of Italy, North Africa, and a small foothold in southern Spain. Justinian's reconquests have been criticised by historians for overextending his realm and setting the stage for the Muslim conquests, but many of the difficulties faced by Justinian's successors were due not just to over-taxation to pay for his wars but to the essentially civilian nature of the empire, which made raising troops difficult.

Where did most of the invasion in the Eastern Roman Empire take place?
the Balkans