Marcus Aurelius became emperor after Hadrian. He was forced into warfare on the northern frontiers, where it was essential that the emperor himself led the campaigns.
One of the notable features of his reign was he promoted army officers and civilian administrators based on merit, rather than on noble birth. Marcus appointed capable people to posts best suited to their abilities. He laid the foundations of social mobility and broadened the recruitment base for the armies, allowing for greater future flexibility.
Marcus Aurelius is also remembered for his philosophical meditations.
Source: Marcus Aurelius
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