During the reign of Constantine I, Constantine decided that the empire was too large to defend by one person. So instead, he split the empire in two, with the western half still called "Roman Empire" and still having Rome as it's capital, while the eastern half, christened "Byzantine Empire", with it's capital as Constantinople, named after Constantine himself. Under this set up, the two can work independently from each other and protect the lands well.
However, there were problems, including the weakening of the western empire because of barbaric invasions, as well as civil wars between the two empires. The split was the start of the decline of the western half of the Roman Empire.
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Inside the Year of 4 emperors, 4 emperors ruled in succession, with three of them dying within the year, months, weeks, or even days in ruling. The civil war was brutal, and many people died in the ensuing fights between the generals. The four generals divided their territories, with one owning Rome, parts of Africa and present-day Turkey, another ruling over northern Israel and North of Greece, the third ruling over most of Spain, and the last ruling Southern Britain and French Gaul.
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