For over half a millennium Rome spread its civilization by force of arms across much of Europe, North Africa and the Near East. Why then, in the 5th century CE , did Roman power collapse?
Rome had been under pressure for some time. Germanic tribes had been pushing at the frontiers of the empire for centuries. They in turn were coming under pressure from warlike peoples spreading out of Asia, such as the Huns. The Roman empire relied more and more on the army, and this encouraged a succession of generals to declare themselves emperor. The result was political instability and civil war. The cost of defending the empire led to crippling taxation and inflation. Trade and agriculture suffered, and famine and epidemics further damaged the fabric of society. In the 4th century the seat of Roman power moved to a new capital in the east, Constantinople (now Istanbul). Rome itself was left to its fate. In 410 CE it was sacked by the Visigoths, and the last Roman emperor in the west fell in 476.
Some empires have ended with catastrophic speed. Alexander the Great of Macedon destroyed the Persian empire in the 330s BCE . The Aztecs in Mexico and the Incas in South America were extinguished within two or three years of the arrival of the Spanish in the early 16th century (see here ).
Over some 7,500 years, successive ruling dynasties in China broke down for various reasons – internal rivalry, peasant revolts, foreign invasion – but each new dynasty from the Qin onwards inherited both the imperial territory and the administrative power structure of its predecessor, and thus China remained as a united empire throughout most of its history. Contrast this with the end of the Mauryan empire in India, which failed to bequeath such robust imperial institutions. This is partly because the Chinese system became meritocratic, and capable individuals ran the empire even under weak or unpopular emperors, whereas, after the death of the popular and charismatic Asoka the Great in 232 BCE , the Mauryan empire went into decline and had fallen within fifty years.
The reasons why empires fall tend to be related to the challenges all empires face. To keep local populations loyal, maintain military strength, encourage economic prosperity and build an administrative structure strong and flexible enough to rule distant territories are all difficult challenges, and failure to achieve any one of these can lead to disintegration or collapse.