So, how did one of the greatest empires in history collapse? Eventually, as they accumulated more territory, the Roman Empire became too large to keep under a single Caesar's control. The Empire was divided into a tetrarchy (rule by 4), which was the first step to a divided empire. Diocletian (one of the Caesars) was the one who issued this tetrarchy into order. Then, in 305A.D., after suffering from illness the year before, Diocletian did the unthinkable and became the first Caesar to retire from office. This shocking abdication of power further divided Rome. As fighting broke out, another controversial move was made by the Roman leaders: the capital was changed from Rome to the ancient Greek city of Byzantium (later called Constantinople, after Constantine). An ensuing schism eventually ended the Roman Empire, splitting it into Byzantine East and Latin West. During the long struggle with invaders from the north, cities in the Western Roman Empire were abandoned by frightened inhabitants who sought refuge in the countryside. The population dwindled from 1.5 million to about 300,000. Magnificent temples, palaces, and amphitheaters were torn down, and the stone, marble, and concrete was used to erect fortifications to keep the invaders out. The effort was useless. Once-proud cities were overrun, and their art treasures, destroyed or carried off. Following this is the Dark Ages.