[Little Bill]

The Justinian plague led to the reduction of population in the Byzantine half of the empire, and to its later inability to resist the Persians and later the Moose Limbs.

The Magna Carta was in the 12th century well before the Great Plague. Europe didn't recover from the J-Plague until the time of John, look at population density in the Doomsday book, 1086, and 1345.

[Antoninus]

The Justinian plague led to the reduction of population in the Byzantine half of the empire, and to its later inability to resist the Persians and later the Moose Limbs.

Actually, it weakened Persia, too. If memory serves, Chosroes I of Persia ended his highly successful campaigns against the Roman Empire in the early 540s and made a long-term peace partly as a result of the plague.

The Romans were actually well able to defend themselves against Persia in the late 6th century. It was only after the civil war early 7th century surrounding the usurpation of Phocas that the Persians were again able to successfully strike into the heart of the Empire, conquering Egypt, Palestine, and much of Syria. But even then, they were driven back and utterly defeated by the Emperor Heraclius in the late 620s. Of course, this effort exhausted both the Romans and Persians, leaving them easy prey for an evil force welling up from the Arabs to the south...