The arrival of the Seljuks

One possible explanation for the disintegration of the Byzantine Empire is the permanent settlement of Anatolia by the Seljuks, a Turkic people.

It is worth remembering that the Byzantines had faced invasion in Asia Minor for centuries. In the seventh and eighth centuries, in particular, first the Persians and then the Arabs launched major offensives into the region. Yet these invaders ultimately failed to establish themselves in Byzantine territory.

However, a period of civil war in the late eleventh century enabled the Turks to make huge inroads into Byzantine territory. In many places, usurpers used mercenary Turkish troops to occupy strategic towns, only for those mercenaries to take the towns for themselves when the usurpers had departed. Thus by 1095, virtually the whole of Asia Minor, comprising about 70% of the Byzantine Empire, had been lost.

Although the three competent Komnenian emperors, especially Manuel I Komnenos (r.1143-1180), may have had the power to expel the outnumbered Seljuks, several factors combined to ensure that they never did so. Alexios was unable to derive much of the expected benefit from the First Crusade, though it did at least help him to recover Nicaea and western Asia Minor. It has even been argued that it was never in the interests of the Komnenoi to expel the Turks, as the expansion back into Anatolia would have meant sharing more power with the feudal lords, thus weakening their power. If this is so, it is deeply ironic, as re-conquering Anatolia may have saved the Byzantine Empire in the long run.

No emperor after the Komnenian period was in a position to expel the Turks from Asia Minor, while the preoccupation of the Nicaean emperors with the attempt to recover Constantinople meant that resources were diverted away from Asia Minor and towards the west. The result was a weakenining of the Byzantine defences in the region, which when combined with insufficient resources and incompetent leadership lead to the complete loss of all the empire's Asian territory to the Turks by c.1400. Yet ultimately, any explanation of the change from Byzantine to Turkish rule in Asia Minor should ideally give weight to the strengths of the victors as well as the weaknesses of the defeated. Once this is done, it is hard to assess the responsibility of individual factors for the changes that occurred in Asia Minor during this period.

The structure of the military

Another major factor in the decline of the Byzantine empire may have been the disintegration of its traditional military system, the 'theme' system, which had supplied large numbers of troops for the empire in earlier centuries.

The first advantage of the theme system had been its numerical strength. It is thought that the Byzantine field army under Manuel I Komnenos (r. 1143-1180) had numbered some 40,000 men. However, there is evidence that the thematic armies of earlier centuries had provided the empire with a numerically superior force. The army of the theme of Thrakesion alone had provided about 9,600 men in the period 902-936, for example. Furthermore, the thematic armies had been stationed in the provinces, and their greater independence from central command meant that they were able to deal with threats quickly at a local level. This, combined with their greater numbers, allowed them to provide greater defense in depth.

The other key advantage of the theme system was that it had offered the Byzantine state good value for money. It provided a means of cheaply mobilising large numbers of men. The demise of the system meant that armies became more expensive in the long run, which reduced the numbers of troops that the emperors could afford to employ. The considerable wealth and diplomatic skill of the Komnenian emperors, their constant attention to military matters, and their frequent energetic campaigning, had largely countered this change. But the luck of the empire in having the talented Komneni to provide capable leadership was not a long term solution to a structural problem in the Byzantine state itself. After the death of Manuel I Komnenos in 1180, the Angeloi had not lavished the same care on the military as the Komneni had done, and the result was that these structural weakness manifested themselves in military decline. From 1185 on, Byzantine emperors had found it increasingly difficult to muster and pay for sufficient military forces, while their incompetence had exposed the limitations of the entire Byzantine military system, dependent as it was on competent personal direction from the emperor. The culmination of the empire's military disintegration under the Angeloi had therefore been reached on 13 April 1204, when the armies of the Fourth Crusade sacked Constantinople and dismantled the Byzantine Empire.

Thus, the problem was not so much that the Komnenian army was any less effective in battle (the thematic army's success rate was just as varied as that of its Komnenian counterpart); it is more the case that, because it was a smaller, more centralised force, from the twelfth century on the army required a greater degree of competent direction from the emperor in order to be effective. Although formidable under an energetic leader, the Komnenian army did not work so well under incompetent or uninterested emperors. The greater independence and resiliance of the thematic army had provided the early empire with a structural advantage that was now lost.

For all of the reasons above, it is possible to argue that the demise of the theme system was a great loss to the Byzantine empire. Although it took centuries to become fully apparent, one of the main institutional strengths of the Byzantine state was now gone. Thus it was not the army itself that was to blame for the decline of the empire, but rather the system that supported it. Without strong underlying institutions that could endure beyond the reign of each emperor, the state was extremely vulnerable in times of crisis. Byzantium had come to rely too much on individual emperors.

The fall of the Byzantine Empire

After the sack of Constantinople in 1204, three Byzantine successor states were established. These states included the Empire of Nicaea, the Empire of Trebizond, and the Despotate of Epirus.

The Empire of Nicaea, controlled by the Palaiologan dynasty, managed to reclaim Constantinople from the Latins in 1261 and defeat Epirus. This led to a short lived revival of Byzantine fortunes under Michael VIII, but the war-ravaged empire was ill-equipped to deal with the encircling enemies that now surrounded it. Much of Constantinople lay in ruins; the army was desperately short of funds; Italian merchants and their ships dominated the empire's sea-lanes; the economy was in decline; the empire's ancient frontiers had been overrun; the provinces were in disarray.

Civil war racked the empire during the 14th century. The Asian provinces were lost to the Turks, while the Serbians and Bulgarians conquered the empire's remaining territory in Europe. For a while, the empire survived simply because the Turks of Anatolia were too divided to attack. However, the unifying influence of Osman I (1258–1326) allowed the newly founded Ottoman Empire to deprive the Byzantines of all but a handful of port cities.

The Emperors appealed to the west for help, but the Pope would only consider sending aid in return for a reunion of the Eastern Orthodox Church with the See of Rome. Church unity was considered, and occasionally accomplished by imperial decree, but the Orthodox citizenry and clergy refused to accept Roman Catholicism and shunned the Latin Rite. Some western mercenaries arrived to bolster the Christian defence of Constantinople, but most Western rulers, distracted by their own affairs, did nothing as the Ottomans picked apart the remaining Byzantine territories.

The Turks had previously considered Constantinople not worth the considerable effort of conquest, but with the advent of cannon, the walls (which had been impenetrable to assault for over 1,000 years) no longer offered adequate protection against a siege.

Constantinople by this stage was underpopulated and dilapidated. The population of the city had collapsed so severely that it was now little more than a cluster of villages separated by fields. In March 1453, an Ottoman army of 85,000 men led by Sultan Mehmet II laid siege to the city. Despite a desperate last-ditch defense of the city by the massively outnumbered Christian forces (7,000 men, 2,000 of whom were foreign mercenaries), Constantinople finally fell to the Ottomans after a two-month siege by on Tuesday May 29, 1453.

The last Byzantine emperor, Constantine XI Paleologus, was last seen entering deep into the fighting of a over-whelmingly outnumbered civilian army, against the invading Ottomans on the ramparts of Constantinople. Mehmed II also conquered Mistra in 1460 and Trebizond in 1461.

Mehmed and his successors, continued to consider themselves proper heirs to the Byzantines until their own demises early in the 20th century. By the end of the century, the Ottoman Empire had established its firm rule over Asia Minor and most of the Balkan peninsula.