hirelings. After the Battle of Adrianople, the Gothic foederati, by the Treaty of 382, were allowed to remain with the Empire intact, which reversed the centuries-old Roman policy of destroying barbarian enemies by killing them all, selling them or incorporating them into the Roman army by scattering them across units. The hospitalitas system granted a third of the land (or fees) of a region to barbarians who had invaded and occupied those lands assigned to them. In return, these people declared loyalty to the Emperor and provided military support, whilst retaining their independence. If the theory is correct, Germanic peoples may have been resident in Britain before and after these reforms. One thing led to another to create a spiral. The policy of substituting mercenaries who were paid in gold which should have gone to support the professional standing army and accommodation to their presence spelled the doom of the Western Empire. The federates, operating from within the boundaries of the Empire, eventually became the new landlords, as there was no professional Roman army to subdue them. Ancient authors bitterly comment on this series of policy mistakes. The East which relied far less on mercenaries escaped Britain's fate. The Britons in the decades after 410 repeated the mistakes the emperors had done with the Visigoths, Burgundians, Sueves, Vandals, and Franks – they invited mercenaries to protect them, then those same mercenaries subsequently took control over Britain. Plague of Justinian The population of Britain may have decreased by between 1.5 and 3 million after the Roman period, perhaps caused by environmental changes (the Late Antique Little Ice Age) and, subsequently, by plague and smallpox (around 600, the smallpox spread from India into Europe). It is known that the Plague of Justinian entered the Mediterranean world in the 6th century and first arrived in the British Isles in 544 or 545, when it reached Ireland. It is estimated that the Plague of Justinian killed as many as 100 million people across the world. As a result, Europe's population fell by around 50% between 550 and 700. The later medieval Welsh Lludd and Llefelys mentions a series of three plagues affecting the British of London. Anglo-Saxon settlement According to research led by University College London, Anglo-Saxon settlers could have enjoyed a substantial social and economic advantage over the native Celtic Britons who lived in what is now England, for more than 300 years