The extraordinary successes of Justinian's military campaigns obscured the omissions . The danger which threatened the Empire through the immigration of the Slavs was recognised too late ; the peace made with Persia was bought for too high a price and did not last long . The religious conflict between the Monophysitic provinces (in particular Egypt and Syria) and the Orthodox ones put the Empire to a severe test which from its violence may be compared with the wars between Protestants and Catholics in the 16th. and 17th. centuries in the west . The achievement of a balance between the social classes and the revival of production and trade failed because the monetary requirements prevented any sensible limit on the collection of taxes . After Justinian's death in 565 , the Empire disintegrated within a few decades .

In 602 , a general , Phocas , came to power by a coup . Under his regime of terror which led to a civil war , Byzantium slid into a catastrophe . In the Balkans , Slavonic and Bulgarian tribes established their own states , while in Asia Minor the Persians stood before the gates of Constantinople . The age of imperial greatness was irrevocably over , although the Empire did recover again from this almost impossible position to become a considerable power factor in the early Middle Ages .

In 610 , Herakleios , the son of the governor of Carthage , a Byzantine military province in North Africa , arrived and deposed Phocas . This further usurpation marks the beginning of a new , transformed Byzantine state . Herakleios needed twelve years in order to prepare the remains of the Empire for a counter - offensive . He was helped in this by Byzantine diplomacy with its rich tradition and the progress made in military techniques , but also a reform of the administration and command structures . The reasons for his huge success are unclear on account of the poor position with regard to sources , but he apparently recognised the signs of the time and was an excellent strategist . In order not to expend himself in a war on two fronts , he made peace with the enemies in the West . Avars and Slavs had besieged Constantinople for months in 618 and 626 simultaneously with the Persians ; admittedly , their fighting power had suffered so much as a result that Herakleios was able to negotiate tolerable terms for Byzantium . Instead of now fighting skirmishes with the Persians for each individual tract of land , Herakleios moved his headquarters to the Caucasus and attacked the Sassanids at the centre of their power . In 627 , the old main enemy had to accept its last , final defeat .

Herakleios led Byzantium to a new climax . Admittedly , the territory which he had now won was reduced to a third of its size even during the period of his rule . The year in which he had started the Byzantine counter - offensive was the year of the Hegira , the beginning of the Islamic era , 622 A.D. The development of the third monotheistic religion was so rapid that it completely escaped the Byzantine spies . The first attack in 630 was regarded in Constantinople as an unimportant border incident . In 638 Jerusalem was Moslem ; in 640 Syria became part of the Caliphate ; in 642 Egypt . Within six years , Herakleios lost his economically strongest , also intellectually rich provinces , and war on two fronts remained the fate of Byzantium : in the north - west against the Slavs and Bulgarians , in the south - east against the Arabs . For 200 years the confrontation maintained the character of a struggle for very existence of the empire .

The Byzantines succeeded in adapting to the requirements of a permanent state of war . Justinian I had already abolished the separation of civil and military power typical for the late Roman Empire in the newly acquired provinces of Italy and Tunisia . His immediate successors , especially Maurice , continued spinning this thread and were thus able to record some successes against the Slavs . Herakleios also further expanded the theme system . It reached its final form under his successors . A theme was an administrative district which was under the rule of a military commander (strategos) . The term thema may probably be traced back to the bundle of files in which soldiers within a province were recorded . The Strategos , that is a Byzantine title , was the military commander and administrative head in one . He was able to act independently in many things and did not waste time obtaining permission beforehand . The short decision-making paths made the state elastic without seriously threatening the Emperor's claim to final leadership .

The governor levied the taxes for his district and used the money for the expenditure incurred there . The Emperor financed the central bureaucracy and the palace from the surpluses which the state monopoly enterprises earned , as well as from customs duties and special levies . The pay for the soldiers in the relatively small but superbly trained army was saved by means of the Stratiot system . A stratiot received land and from what he earned from that he had to pay for his equipment and his livelihood . He was employed for the defence of the area in which this piece of land lay and therefore had - in contrast to a purely mercenary army - a personal interest in the outcome of a battle . In addition , these soldiers' settlements served the population policy . There were repeated compulsory resettlements of whole tribes ,Slavs , Armenians , Cypriots , etc . were transplanted to areas which had become depopulated as a result , for instance , of plague epidemics or military campaigns . This was also intended to take people away from the enemy .

In addition to the stratiot units came the fleet which was provided by two maritime themes . The Byzantine navy the decisive means of warfare , already because no other state in the Middle Ages had such a long coast . Against the Arabs , who quickly acquired an effective fleet of their own through the Monophysites in Syria , the Byzantines were able to use an invention which was kept so secret that its exact composition is unknown even today . This was what was known as 'Greek fire' by which a petroleum mixture was thrown onto the sea by a siphon and then ignited so that the enemy ship caught fire . Without this weapon , Constantinople would hardly have survived the frequent Moslem sieges . The fact that the Empire collapsed in the 13th. century under the onslaught of the Crusaders was due above all to the decline of the fleet , the result of the insidious financial haemorrhage .

Among the curious features of Byzantine history is the linking of efficiency and iconoclasticism in this period of resistance .