The Byzantine Empire experienced several cycles of growth and decay over the course of nearly a thousand years, including major losses during the Arab conquests of the 7th century. However, modern historians generally agree that the start of the empire's final decline began in the 11th century.
In the 11th century the empire experienced a major catastrophe in which most of its heartland territory in Anatolia was lost to the Seljuk Turks following the Battle of Manzikert and ensuing civil war. At the same time, the empire lost its last territory in Italy to the Norman Kingdom of Sicily and faced repeated attacks on its territory in the Balkans. These events created the context for emperor Alexios I Komnenos to call to the west for help, which led to the First Crusade. However, economic concessions to the Italian republics of Venice and Genoa weakened the empire's control over its own finances, especially from the 13th century onward, while tensions with the West led to the Sack of Constantinople by the forces of the Fourth Crusade in 1204 and the dismemberment of the empire.
Although a number of small Byzantine successor states survived and eventually reclaimed Constantinople in 1261, the empire had been severely weakened. In the longer term, the rise of Turkish power in Anatolia eventually gave rise to the Ottoman Empire which rapidly conquered the former Byzantine heartland over the course of the 14th and 15th centuries, culminating in the Fall of Constantinople to the army of Sultan Mehmed the Conqueror in 1453.
Further information: History of the Byzantine Empire
The most significant events generally agreed by historians to have played a role in the decline of the Byzantine empire are summarised below:
Probably the most important single cause of Byzantium's collapse was its recurrent debilitating civil wars. Three of the worst periods of civil war and internal infighting took place during Byzantium's decline. Each time, these civil wars coincided with a catastrophic reduction in Byzantine power and influence, which was never fully reversed before the next collapse.
The period from 1071 to 1081 saw eight revolts:
This was followed by a period of secure dynastic rule by the Komnenos dynasty, under Alexios I (1081-1118), John II Komnenos (1118-43) and Manuel I Komnenos (1143-1180). Cumulatively, these three emperors were able to partially restore the empire's fortunes, but they never were able to fully undo the damage caused by the instability at the end of the 11th century, nor return the empire's frontiers to those of 1071.
The second period of civil war and collapse took place after Manuel's death in 1180. Manuel's son Alexios II Komnenos was overthrown in 1183 by Andronikos I Komnenos, whose reign of terror destabilised the empire internally and led to his overthrow and death in Constantinople in 1185. The Angelos dynasty which ruled Byzantium from 1185 to 1204 has been considered one of the most unsuccessful and ineffectual administrations in the empire's history. During this period, Bulgaria and Serbia broke away from the empire, further land was lost to the Seljuk Turks. In 1203, the imprisoned former emperor Alexios IV Angelos escaped jail and fled to the west, where he promised the leaders of the Fourth Crusade generous payment if they would help him regain the throne. These promises later proved to be impossible to keep; in the event, the dynastic squabbling between the weak and ineffectual members of the Angelid dynasty brought about the Sack of Constantinople; Constantinople was burned, pillaged and destroyed, thousands of its citizens were killed, many of the surviving inhabitants fled, and much of the city became a depopulated ruin. The damage to Byzantium was incalculable; many historians point to this moment as a fatal blow in the empire's history. Although the empire was reformed in 1261 by the recapture of the city by forces from the Empire of Nicaea, the damage was never reversed and the empire never returned to anywhere near its former territorial extent, wealth and military power.
The third period of civil war took place in the 14th century. Two separate periods of civil war, again making extensive use of Turkish, Serbian and even Catalan troops, often operating independently under their own commanders, and often raiding and destroying Byzantine lands in the process, ruined the domestic economy and left the state virtually powerless and overrun by its enemies. Conflicts between Andronikos II and Andronikos III, and then later between John VI Kantakouzenos and John V Palaiologos, marked the final ruin of Byzantium. The Byzantine civil war of 1321–1328 allowed the Turks to make notable gains in Anatolia and set up their capital in Bursa 100 kilometers from Constantinople the Byzantine's capital. The civil war of 1341–1347 saw exploitation of the Byzantine Empire by the Serbs, whose ruler took advantage of the chaos to proclaim himself emperor of the Serbs and Greeks. The Serbian king Stefan Uroš IV Dušan made significant territorial gains in Byzantine Macedonia in 1345 and conquered large swathes of Thessaly and Epirus in 1348. In order to secure his authority during the civil war, Kantakouzenos hired Turkish mercenaries. Although these mercenaries were of some use, in 1352 they seized Gallipoli from the Byzantines. By 1354, the empire's territory consisted of Constantinople and Thrace, the city of Thessaloniki, and some territory in the Morea.
Main article: Byzantine army
Main article: Komnenian army
The disintegration of the Byzantine Empire's traditional military system, the 'theme' system, played a role in its decline. Under this arrangement, which was in its heyday from circa 650 to 1025, the empire was divided into several regions which contributed locally raised troops to the imperial armies. The system provided an effective means of cheaply mobilizing large numbers of men, and the result was a comparatively large and powerful force – the army of the theme of Thrakesion alone had provided about 9,600 men in the period 902–936, for example. But from the 11th century onwards, the theme system was allowed to decay. This played a major role in the loss of Anatolia to the Turks at the end of that century.
In the 12th century, the Komnenian dynasty re-established an effective military force. Manuel I Komnenos, for example, was able to muster an army of over 40,000 men. However, the theme system was never replaced by a viable long-term alternative, and the result was an empire that depended more than ever before on the strengths of each individual emperor or dynasty. The collapse of imperial power and authority after 1185 revealed the inadequacy of this approach. After the deposition of Andronikos I Komnenos in 1185, the dynasty of the Angeloi oversaw a period of military decline. From 1185 onwards, Byzantine emperors found it increasingly difficult to muster and pay for sufficient military forces, while the failure of their efforts to sustain their empire exposed the limitations of the entire Byzantine military system, dependent as it was on competent personal direction from the emperor.
Despite the restoration under the Palaiologoi, Byzantium was never again a great power on the scale of the past. By the 13th century, the imperial army numbered a mere 6,000 men. As one of the main institutional strengths of the Byzantine state, the demise of the theme system left the empire lacking in underlying structural strengths.
As far back as the invasion of Africa by Belisarius, foreign soldiers were used in war. While foreign military intervention was not an all together new occurrence, the reliance on it, and its ability to damage political, social, and economic institutions were dramatically increased in the 11th, 13th, 14th, and 15th centuries. The 11th century saw increasing tensions between Courtly, and Military factions. Until the mid 11th century the empire had long been under the control of the Military Factions with leaders such as Basil II, and John I Tzimiskes, however the crisis of Basil II's succession led to increasing uncertainty in the future of politics. The army demanded Constantine VIII's daughters ascend to the throne by virtue of their relation to Basil II, leading to a number of marriages, and increasing power for the Courtly faction. This culminated after the failed Battle of Manzikert. As civil wars broke out, and tensions between courtly, and military factions reached a zenith, the demand for soldiers led to the hiring of Turkish mercenaries. These mercenaries aided in the Byzantine loss of Anatolia by drawing more Turkish soldiers into the interior of the empire, and by giving the Turks an increasing presence in Byzantine politics. These interventions also led to further destabilization of the political system.
Reliance on foreign military intervention, and sponsorship for political motives, continued even during the Komnenoi Restoration, Alexius I used Turkish mercenaries in the civil wars he participated in with Nikephoros III Botaneiates. In 1204, Alexios IV Angelos relied on Latin soldiers to claim the throne of Byzantium, leading to the sack of Constantinople, and the creation of the successor states.
Economic concessions to the Italian Republics of Venice and Genoa weakened the empire's control over its own finances, especially from the ascension of Michael VIII Palaiologos in the 13th century onward. At this time it was common for emperors to seek sponsorship from Venice, Genoa, and the Turks. This led to a series of disastrous trade deals with the Italian states; drying up one of the empire's final sources of revenue. This further led to competition between Venice, and Genoa to get emperors on the throne who supported their respective trade agenda to the detriment of the other, adding another level of instability to the Byzantine political process.
By the time of the Byzantine–Genoese War (1348–49), only thirteen percent of custom dues passing through the Bosporus strait were going to the Empire. The remaining 87 percent was collected by the Genoese from their colony of Galata. Genoa collected 200,000 hyperpyra from annual custom revenues from Galata, while Constantinople collected a mere 30,000. The loss of control over its own revenue sources drastically weakened the Byzantine empire, hastening its decline. At the same time, the system of Pronoia (land grants in exchange for military service), became increasingly corrupt and dysfunctional by the later empire, and by the 14th century many of the empire's nobles were not paying any tax, nor were they serving in the empire's armies. This further undermined the financial basis of the state, and placed further reliance on unreliable mercenaries, which only hasted the empire's demise.
Emperor Michael VIII Palaiologos signed a union with the Catholic church in the 13th century in the hope of staving off western attack, but the policy was unsuccessful. The empire's western enemies soon resumed attacking the empire, while the social divisions the deeply unpopular union created inside the empire were damaging to Byzantine society. The controversy over church union failed to provide the empire with any lasting benefit, while the prisons were soon full of dissenters and Orthodox clergy. This undermined the legitimacy of the Palaiologos dynasty and further facilitated social divisions, which were ultimately to play a role in the loss of Anatolia to the Ottoman Turks.
Byzantine envoys presented themselves at the Second Council of Lyons 24 June 1274. On the fourth session of the Council the formal act of union was performed, however with Pope Gregory's death (January, 1276), the hoped for gains did not materialise.
While the union was opposed at all levels of society, it was especially opposed by the greater populace, led by the monks and the adherents of the deposed Patriarch Arsenios, known as the Arsenites. One of the chief anti-unionist leaders was Michael's own sister Eulogia (aka Irene), who fled to the court of her daughter Maria Palaiologina Kantakouzene, Tsarina of the Bulgars, from where she intrigued unsuccessfully against Michael. More serious was the opposition of the sons of Michael of Epirus, Nikephoros I Komnenos Doukas and his half-brother John the Bastard: they posed as the defenders of Orthodoxy and gave support to the anti-unionists fleeing Constantinople. Michael at first responded with comparative leniency, hoping to win the anti-unionists through persuasion, but eventually the virulence of the protests led him to resort to force. Many anti-unionists were blinded or exiled. Two prominent monks, Meletios and Ignatios, were punished: the first had his tongue cut out, the second was blinded. Even imperial officials were harshly treated, and the death penalty was decreed even for simply reading or possessing pamphlets directed against the Emperor. "From the intensity of these disorders, tantamount almost to civil wars," concludes Geanakoplos, "it might appear that too great a price had been paid for the sake of union."
The religious situation only worsened for Michael. The Arsenite party found widespread support amongst the discontented in the Anatolian provinces, and Michael responded there with similar viciousness: according to Vryonis, "These elements were either removed from the armies or else, alienated, they deserted to the Turks". Another attempt to clear the encroaching Turkmen from the Meaender valley in 1278 found limited success, but Antioch on the Maeander was irretrievably lost as were Tralles and Nyssa four years later.
On 1 May 1277, John the Bastard convoked a synod at Neopatras that anathematized the Emperor, Patriarch, and Pope as heretics. In response, a synod was convoked at the Hagia Sophia on 16 July where both Nikephoros and John were anathematized in return. John called a final synod at Neopatras in December 1277, where an anti-unionist council of eight bishops, a few abbots, and one hundred monks, again anathematized the Emperor, Patriarch, and Pope.
Though the Crusades assisted Byzantium in driving back some of the Turks, they went far beyond the military assistance envisaged by Alexios I. Instead of following the strategic necessities of the war against the Turks, the Crusaders were focussed on the quest of re-conquering Jerusalem, and instead of returning territory to Byzantium, the Crusaders established their own principalities, becoming a territorial rival to Byzantine interests in their own right.
This was true already during the Third Crusade, which induced emperor Isaac II Angelos to make a secret alliance with Saladin to impede the progress of Frederick Barbarossa, but open conflict between Crusaders and Byzantium erupted in the Fourth Crusade, resulting in the Sack of Constantinople in 1204. Constantinople was now itself a Crusader state, known as the Latin Empire in historiography, but from the Greek perspective as Frankokratia or "rule of the Franks". Vestiges of imperial power were preserved in minor principalities, the Nicaean Empire, Trebizond and Epirus. Much of the Nicaean Emperors' efforts now went into combating the Latins, and even after Constantinople was returned to Greek rule under the Palaiologoi in 1261, the Empire exerted much of its efforts into defeating its Latin neighbours, contributing to the eventual failure of the Crusades by 1291.
Main article: Byzantine–Seljuq wars
Main article: Byzantine-Ottoman wars
See also: Ottoman claim to Roman succession
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 weakening of the Byzantine defenses in the region, which, when combined with insufficient resources and incompetent leadership, led to the complete loss of all the empire's Asian territory to the Turks by 1338.
The disintegration of the Seljuk Turks led to the rise of the Ottoman Turks. Their first important leader was Osman I Bey, who attracted Ghazi warriors and carved out a domain in north-western Asia Minor. Attempts by the Byzantine Emperors to drive back the Ottomans were unsuccessful, and ceased in 1329 with the Battle of Pelekanon. Following a number of civil disputes in the Byzantine Empire, the Ottomans subjugated the Byzantines as vassals in the late 14th century and attempts to relieve this vassal status culminated in the Fall of Constantinople in 1453.