I've been reading Walter Kaegi's book about this era recently, and one of the things that he emphasizes is the Roman predilection for relying on defensive strong points and only committing to battle when suitable terrain can be occupied, this in stark contrast to the Arabs' lightning campaigns and mobility.
So in some ways this seems comparable to Constantine's reforms and it is understandable given the demographic decline and desperate financial situation after plague and constant war of the past century. The Romans best field army was far away from the action in the area of Constantinople and there were not large enough forces along the Palestinian and Syrian frontier to counter a serious invasion. This strategy also contrasts with the Persians' reaction to the Muslim attacks: mustering large (but often ill-prepared) armies for immediate retaliation. As in Alexander the Great's campaigns the Persians did not have a well-developed network of static defenses and so they were pretty much doomed after major defeats in the field; all this in spite of the Persians employing slow developing, caustious tactics in battle like the Romans.A broad strategy of mobile defense in depth characterized Byzantine military operations and defense efforts in Palestine and Syria. This resulted from the small-scale frequent nomadic raids that did not normally require the mustering and dispatch of large intervention forces for the defense of the empire. The empire lacked the proper topography or sufficient manpower to resort to a frontal/forward defense strategy, which would have committed the bulk of troops along the empire's periphery to stop hostile incursions before they resulted in serious damage to the terrain, structures, agriculture, and civilian population of the exposed areas.
The fairly small forces available in Palestine, Egypt, and Syria were also of poor quality and could delay such a dangerous enemy at best. Also the Christian Arabs were often unreliable, even defecting to the Muslims at times.Fortresses provided fixed points for defense... but Byzantine fortresses were not located in some manner that interlocked them so that no one could take or neutralize them except sequentially one by one. Only modest numbers of light forces, in the hundreds of troops, were stationed at scattered posts in a frontier zone of considerable depth (100+ miles probably) itself.
The Byzantines were prepared in Syria and Palestine to confront small scale penetrations by having mobile garrisons of local troops under duces, but local Arab allies did most of the patrolling and fighting. The central intervention forces who were best able to counter rare, large scale penetrations were located near Constantinople and behind upper Mesopotamian frontiers with Persia...
Muslim strategy and operations were well suited for this defense-in-depth of the Byzantines and took good advantage of it.
Basically the Romans were trading land (very valuable land at that) for time to regroup and not risking decisive engagements, the exception was Yarmuk and there weren't many after that. So, considering the state of the empire and its military at the beginning of the conflict on one hand and the fate of its former rival Persia against the same enemy, is it justified to actually consider the Roman defensive strategy a success instead of utter failure (as it usually is), since it preserved the imperial core are in the long term and allowed for recovery and even reconquest in the future?This strategy of defense-in-depth, because it avoided decisive tests whenever possible, probably, although at great civilian and territorial costs, contributed to the survival of at least one part of the Byzantine Empire.