Answer:
The Roman Empire was once a superpower. Back in the days of the early 2nd century, Emperor Trajan stretched the kingdom's territory to its maximum. After that, how to secure the frontier had become an issue that all the future emperors had to address. Because most of those emperors were not nearly as capable as Trajan, the Roman Empire was soon in trouble. By the 3rd century, the situation had grown so bad that this once formidable powerhouse was at the brink of self-destruction. During the period from 235 A.D. to 284 A.D. (often called the crisis of the third century, the military anarchy, or the imperial crisis), more than two-dozen emperors came and went. Out-of-control inflation brought the economy to its knees. And foreign tribes continued to harass the borders. Just as things could not get worse for the Roman Empire, relief finally arrived. In November of 284 A.D., Diocletian, a forceful Roman general, seized power and declared himself the new emperor. One of his earliest orders was to split the Roman Empire in two. He kept the eastern part and gave the western half to his colleague, Maximian. Diocletian's decision was bold but practical. He figured that the Roman Empire had simply grown too big over the years to be managed effectively by a single person. In 285 A.D., he named his trusted military friend, Maximian, as a Caesar or a junior emperor, while he himself was named an Augustus or a senior emperor. The following year, Diocletian promoted Maximian to be his equal, so both men held the title of Augustus and ruled the split Roman Empire side-by-side. Diocletian chose the city of Nicomedia (modern day's Izmit, Turkey) to be the capital of his Eastern Roman Empire, whereas Maximian picked Milan to be the capital of his Western Roman Empire. With the kingdom broken into two, Diocletian and Maximian were each responsible for fighting the enemies in their respective territory. As it was no longer necessary to stretch the troops across the entire empire, it was much easier to put down the rebels. Diocletian's daring experiment paid off handsomely. By 293 A.D., Diocletian decided to go a step further and resolve the issue of succession once and for all. That year, both of the senior emperors handpicked their own Caesar. Diocletian chose Galerius, and Maximian selected Constantius. Galerius and Constantius were like apprentices. They did not sit idly waiting for the two senior emperors to die or to retire. Instead, they were each given a sizable territory and had their own capital. Galerius resided at Sirmium (in today's Serbia), and Constantius camped at Trier (in today's Germany). Diocletian called this new power structure tetrarchy or "rule by four."
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