The Romans called the people who lived outside the Roman Empire barbarians. In the 4th century AD the Roman Army had considerable difficulty in stopping these Barbarians from entering the Roman Empire.

The Romans were forced to increase the size of their army. By the end of the 4th century AD it had grown to 600,000 men. Of these, 250,000 were stationed on Rome's northern borders.

Taxation had to be increased to pay for this large army. These taxes were higher than most people could afford and created wide-scale poverty. Some people were forced to sell their children into slavery, while others died of starvation.

Plague also became a problem and during one outbreak people in Rome were dying at the rate of 5,000 a day. The population of the Roman Empire began to fall dramatically and this in turn reduced the numbers of people available to join the army.

The growth of Christianity also created problems for the Roman Empire. Large numbers of Roman citizens became monks and refused to get married and have children. This not only contributed to the fall in population but also reduced Roman tax revenues.

Some Christians claimed that Jesus had preached non-violence. Christians who interpreted the words of Jesus in this way often refused to join the Roman army. Even citizens who were not Christians were reluctant to join, and emperors were forced to recruit slaves, gladiators and criminals. It was also decided to employ barbarian mercenaries. This created long-term problems as the barbarians did not always remain loyal to their Roman paymasters.

Some Roman citizens, upset by heavy taxation and suffering from poverty, formed themselves into an armed resistance group called the Bagaudae. This movement started in Gaul in AD 283 but during the 4th and 5th centuries spread to other parts of the empire. These groups of rebels attempted to set up their own independent states within the empire but the Romans, with the help of barbarian mercenaries, were eventually able to crush them.

However, with the Roman army spending more and more time suppressing its own citizens, it became easier for the Germans to defeat those guarding the frontiers. Between AD 406 and 419 the Romans lost a great deal of their empire to different German tribes. The Franks conquered northern Gaul, the Burgundians took eastern Gaul, while the Vandals replaced the Romans in Hispania.

The Romans were also having difficulty stopping the Saxons, Angles and Jutes overrunning Britain. In AD 410 Emperor Honorius warned the Romans in Britain that they could no longer rely on reinforcements being sent from other parts of the empire.

Alaric, a Visigoths leader, took part in several campaigns under the Romans. However, when he did not receive the expected promotion in the Roman Army, he led his people against the empire. Alaric now demanded that the Visigoths should have their own independent state. In AD 410 Alaric's army was strong enough to enter Rome. Roman slaves joined with the Visigoths in sacking the city. With the slaves joining his army, Alaric now had 40,000 men at his disposal. After roaming around the Roman Empire, the Visigoths eventually decided to settle in Aquitania.

In AD 435 Gaiseric and his Vandal army captured Rome's African province. As Rome relied heavily on African food, this was a serious blow to the survival of the empire. From his base in Carthage, Gaiseric was able to launch sea-raids on Rome. In AD 455 Gaiseric was strong enough to invade Rome and plunder the city.

In the years that followed the Romans also lost southern Gaul. It now became clear that it was only a matter of time before the Roman Army would be completely defeated and the Barbarians who had been recruited into the Roman army began to desert in large numbers.

In AD 476, Odoacer, leader of the barbarian mercenaries in the Roman army, overthrew Emperor Romulus Augustulus, and installed himself as King of Italy. The Roman Empire in the west had come to an end. However, the eastern empire, ruled from Constantinople, continued for another thousand years. It was not until 1453, when Mehmed II captured Constantinople, that the Byzantine Empire, as it became known, ceased to exist.