Barbarian tribes' movements into Roman lands eventually led to the collapse of the Roman Empire in 476. The Roman Empire was too extensive and its borders were too porous. The border garrisons answered to their local commanders—rather than to the emperor in Rome. The Romans vainly tried to defend their territory during their last few centuries, but too many Roman emperors and generals were incompetent. Another problem for Rome was internal squabbling over religion as Christianity became more popular.
The barbarian tribes were often desperate and always dangerous. They were large groups of nomadic peoples who needed new lands in order to survive. Allowing these large bands into the Roman Empire was dangerous because they might not leave. Fighting them was difficult because of their numerical strength and fierce determination.
The few capable emperors of the era were an anomaly. For example, Emperor Marcus Aurelius (121–180) successfully staved off the Parthians and Germans that menaced Rome. But Marcus Aurelius was succeeded by Commodus. The depraved Commodus undid his father's successes by making peace with the Germans. He became increasingly unstable and was eventually assassinated. Rome had nearly twenty different emperors during the chaos that followed Commodus' death. Rome's weak central government made border defense increasingly problematic.
Emperor Diocletian (245–313) attempted to defend the borders more effectively by creating a tetrarchy. This was a group of four men who ruled jointly, each man having his own responsibilities and territories. However, his economic reforms failed, and the tetrarchy did not survive him.
Barbarian invaders did not have to face capable Roman commanders. Rome no longer had great generals like Scipio Africanus, Caesar, and Pompey the Great. The lack of military leadership contributed to a severe military defeat at Adrianople (378).
As Rome became weaker, its army included an increasing number of barbarians. For instance, Attila the Hun's threat to Rome was defeated by a combined army of Romans and Visigoths in 451. However, Rome finally fell in 476.
The Byzantine Empire, the Roman Empire of the East, survived until 1453.