Originally Posted by attica
With regard to Roman Byzantium/Persia - I'll give you a general western historical viewpoint, and I'm sure someone else can give the Islamic version.
Awareness of the history of the Arab conquest is low in the west, and Byzantine history doesn’t seem to be very well remembered by anybody (perhaps because the state has now been entirely wiped out). But in fact the 500 year story of Rome v Persia is one of the titanic struggles of the past.
The Muslim claim with regard to Syria is that the country was burdened by Byzantine taxation so the people were happy to accept new overlords. This is partly true, but it misses out the key reason why. In the years directly preceding the conquest, Byzantium had just fought the equivalent of a World War against Persia. Both sides fielded multi-ethnic armies over huge areas. These armies included various Arab allies, who fought for both sides. For many decades if not centuries, Arab tribes had performed the same role on the Roman Eastern frontier as the Germanic tribes did on the Western – ie as paid mercenaries to supplement the overstretched, undermanned Roman forces. The Arab tribes were immensely proud of this role and must have learned all there was to know about Roman and Persian military tactics during this time.
At the end of this ancient World War the Persians appeared to have finally landed a knock-out blow. They had already swept through Byzantine territory including Syria and - by all military logic - appeared to be on the point of total victory.
The reigning Byzantine Emperor Heraclius knew he could not hope to defeat the Persians head on. Instead he took what limited forces he had remaining to him on a kind of guerrilla war deep into the Persia. He avoided large scale battles where he could but utterly laid waste to the Persian homeland. It still ranks as one of the most astonishing military fight-backs of all time.
As a result the Persians failed to complete their conquest of Byzantium. Heraclius recovered Syria and other Byzantine territories but both empires were left tottering, exhausted, and utterly impoverished. Hence the high taxation and chaotic state of defence.
Meanwhile, the Arab mercenaries were left with no paymasters in either Byzantium or Persia and insufficient forces to stop them taking exactly what they wanted. This is what they did. (Again, there are clear parallels with what had happened on the western frontier 200 years earlier). Syria which had already just been laid waste by the Persians was now captured by the Arabs - and this time it would not be recovered. The spectacular success and relative ease of these attacks (with all the tribes now united by Islam) led to a general assault on wider Byzantine territory and Persia. Persia collapsed totally, while Byzantium miraculously survived (in truncated form) for another 600 years.