subsidies which for years they had been receiving from Heraclius for guarding the frontiers had recently been sus- pended as an economy measure. The forts along that southern border had also been neglected and stripped of their garrisons to enable concentration in the north in face of the Persian danger. This does not mean, however, that this invasion or the simultaneous attack on Mesopotamia were the result of purposeful planning. They were surely designed as casual raids for booty, with no thought of permanent conquest, though this was the outcome to which the logic of events inexorably led.
In 633 one column invaded Syria by the coastal route, two followed the inland caravan route north, and one under Khalid ibn-al-Walid penetrated Mesopotamia. Each de- tachment was approximately 3000 strong at first, but rein- forcements more than doubled this strength as they moved north. Two engagements sufficed to clear southern Palestine of Byzantine troops, and fruitful raiding followed. The emperor Heraclius organized a fresh army at Horns and dispatched it under his brother Theodorus, while abu-Bakr ordered Khalid and his column to join their co-religionists in Syria. This Khalid did by an epic eighteen-day march across the desert, appearing with dramatic suddenness directly in the rear of the improvised Byzantine army. He defeated the Christian Ghassanid forces and pressed through Transjordan to Palestine, where the four reunited detach- ments won a major victory in July 634.
All Palestine now lay open before the invader. For six months random raids were launched in all directions. Another Byzantine army was routed and sought refuge behind the walls of Damascus. Khalid pursued it, isolated the city for six months and in September 635 gained pos- session of it through treachery. With the fall of the metro- polis, total victory was assured. Baalbek and Horns were occupied in 635, and Aleppo, Antioch, Hamah and others early in 636. Only Jerusalem, Caesarea and a few ports
