At what point was the Byzantine Empire's decline irreversible?

The Papal led Latin Crusader conquest of Constantinople in 1204, initiated the irreversible decline of the Byzantine Empire.

When the Crusaders captured Constantinople, they, (rather unfortunately), destroyed the City's 800 year Library and University and may have also temporarily converted the Saint Sophia Cathedral into a Catholic Church.

While the Latin Christian conquest of Constantinople was only 50 years long, it was enough time for the Ottoman Turkish Muslims in the East to gain greater imperialistic momentum as the Turks conquered country after country-(primarily along The Silk Route). By the 1400's, a beleaguered and increasingly anachronistic Byzantine Empire-(whose Administrative Capital was no longer in Constantinople proper, but was actually relocated to the Southern Peloponnesian town of Mystras), was incapable of unilaterally and independently defending itself from a burgeoning Ottoman Islamic Empire in the East.

By May, 1453, Constantinople and the Byzantine Empire fell to the Ottoman Muslim Turkish Sultanate and with it, the conversion of the centuries old Saint Sophia Greek Eastern rite Cathedral into an Ottoman Turkish Muslim Mosque. (And, the Janissary Corps was established whereby many Christian men from Asia Minor and even Constantinople-(who were mostly Greek), were forcibly and thoroughly Ottomanized when having to serve in the higher ranks of the imperial Turkish military; though it should be noted that were also voluntary members of the Janissary corps, including, the Ottomans' most famous Architect, Mimar Sinan).

While the Ottoman Muslim conquest of Christian Constantinople was a devastating loss for the Byzantines-(and for Greek Eastern rite Christianity at large), there were some other external influences which helped "pave the way" for the Ottoman's success in 1453.

The Latin Christian West and the Byzantine Christian East....hated each other-(for a variety of reasons). The sectarian infighting between the Christian peoples was bitterly divisive and ultimately led to the Latin Christian West conquest of the Byzantine Christian East-(albeit for a short while, though with devastating long term consequences for the Byzantines).

Even the Northern Italian city-states, such as Genoa and especially, Venice, greatly benefitted from the Latin West's capture of Constantinople. In doing so, the Northern Italian city states were able to conquer many of Byzantium's Greek island territories, such as, Corfu, Crete and the Cyclades, which were all conquered by the Venetians, as well the Eastern Aegean islands which were conquered by the Genoese. And of course the Aegean island of Rhodes, became a major center for Latin Crusaders, as well as for the Venetians-(as evidenced in the island's widespread and very well preserved Late Medieval and Early Modern architecture).

As one can see, the Papacy, as well the Northern Italian city-states, benefitted greatly from Constantinople's demise and may have purposefully encouraged or even welcomed the Ottoman Turkish Muslim conquest of the Empire's Capital city and its Greek Eastern rite Christian territories. The Latin West's success in conquering Constantinople weakened the power and influence of the Christian East, while simultaneously, maximizing the power and influence of the Latin Christian West which helped to produce major historical and civilizational aftereffects that have survived....well into the Modern Age.

Retrospectively, the Latin Christian West helped turn the legacy of Byzantium...into a historical anecdote reserved for the back pages of World History.

I would recommend checking this wikipedia article to have a basis on which to elaborate. If you check the list of sieges of Constantinople that were successful (after of course the beginning of the Byzantine Empire), you'll find very numerous unsuccessful attempts, and only four successful ones:

• One Byzantine emperor against another
• The Crusaders
• One Byzantyne emperor (supported by Italians and Turks) against another
• The final success of the Turks

These three successful sieges represent the only forces that were able to shake deeply the Byzantine Empire: The Turks, the Crusaders, Italian republics as a support for both of them, and, last but not least, the Byzantine Empire itself. Considering these elements -and the general history of the Byzantine Empire-, I would not consider any of the Arab offensives as a turning point for the Byzantine Empire. I would not speak like that of the battle of Mantzikert. Not even the 4th Crusade, because even if it deeply shakened the Byzantine Empire and forcedit to destroy the new "Frank" kingdoms in the Balkans, ultimately Byzantine Empire took back control of nearly all contested territories.

No, the turning point for the Byzantine Empire was the battle of Ain Jalut, when the Mamluks were able to repel the Mongols: Through this battle the Mamluks, that otherwise had never fought the Byzantine Empire directly, had changed the face of Middle East and liberate the Turks from their two main competitors: the Mongols and the Crusaders. Those two threats put apart (after a few other campaigns), they could put all their potential against the Byzantine Empire.

The thing that really started the fatal downward skid for the Byzantine Empire was the arrival of the Turks from central Asia. They were the best warriors in the area, and just as importantly, were dedicated pastoralists. The best land in the area for their purposes was in central Anatolia, which just happened to be smack dab in the middle of the Byzantine Empire.

For the Byzantines the wealthy coastal areas were more important, but whoever held that central anatolian plain could attack any point on the coast at will. Once the Turks showed up, nearly every military effort of theirs (that wasn't spent on internectine warfare) was spent trying to get that good grazing land in the heart of the empire. The Byzantines could slow them down, sometimes even stop them, but they couldn't turn the situation around.

The Crusades started out as a desparate attempt by the Byzantines to get some help kicking the Turks out of anatolia. Some of these worked better than others. As mentioned by others, the Fourth Crusade backfired completely. But it still took a couple of hundred years (and the invention of cannon) before the Turks finally took Constantinople and extinguished the empire for good.

i would date the "irreversibility" of Turkish power to the Sack of Constantinople in 1204. This resulted in the division of the former Byzantine Empire into the Latin Empire, the Kingdom of Nicea, and several "splinter" groups (e.g. Epirus and Trebizond), with Constantinople a "football" between the first two. Without access to "Latin" mercentaries, the ultimate "winner," Nicea, could field armies only a fraction of the size of Byzantine armies a century earlier.

It's true, as T.E.D. pointed out that the Turks became a threat to the Byzantine Empire by the mid-11th century. Yet the rise of the Komnenos Dynasty, 1081-1185 involved A "rollback" (or "reversal") of Turkish power out of much of western Turkey during thaT period.

From what I recall from historical texts that I've read in my travels, the event that seems to have started the irreversible decline was the Fourth Crusade, when instead of heading to free Jerusalem, the crusading armies attacked and sacked Constantinople. With large parts of the Empire fragmented into Latin states by the armies that had attacked them, it helped the Seljuk Turks (and later the Ottomans) hold the Anatolia area of Asia Minor and strengthened their position for furthur conquests as those states fell.

References: Wikipedia's article on the Fourth Crusade

Personally, for me, the turning point was Manzikert. It wiped out a good portion of the fighting men of the empire, and caused the Seljuks to take the eastern part of Asia Minor, which was a large source of manpower for the emperors under the theme system. So with the threat of a Seljuk invasion, Empire responded with a plea for help to the West, launching the Crusades. The final blow to the Byzantine dominance was the sack of Constantinople by the Fourth Crusade, from which the Empire never really recovered.

A prior article mentions the empire of Justinian (and Leo, by extension), but I would argue that these are 'Roman' empires which are terminated by the eruption of Islam over much of the East Roman Empire.

This was a pretty traumatic event which led to some serious results. Among them, the abandonment of Latin, abandonment (with some exceptions) of universal pretensions in the western Mediterranean, acceptance of defence against the Muslim and recovery of the Holy Places.

And then there was iconoclasm on the cultural front. If you look carefully, there is a discernible shift in Weltanschauing in the 8th century in the lands ruled from Byzantium which may be called the birth of the Byzantine Empire and civilization and civilizing efforts through the Balkans and beyond Now, I am aware that my comments mimic those of Arnold Toynbee, but this is one case where his old thesis does strike true.

Of course, the Byzantine state enjoyed considerable success under generals like Nicephorus Phocas and Johm Tzimisces and others long before the time of Basil II (Bulgaroctes) But was it already becoming a feudal state? Surviving records from the maritime Themes show central control, but what was happening in the 'wild west'? Doesn't the epic poem Digines Akrites portray a feudal society on the frontier?

Anyway, I reckon that a feudal army went to Manzikert in 1079 and the rot set in. The Commneni had a very faint chance a hundred years later. Byzantium was still the strongest regional power at the time of the first crusade. But feudalism, both Byzantine and western, prevented any combined effort. Then the battle of Myriokephalon (various spellings) destroyed forever any Byzantine pretensions to power forever.

The empire limped on a for a few more centuries and the civilization may have a few sparks of life left, but I nominate Myriokefalum as the death knell of Byzantiu

I don't think it is possible to idenitify a single point in history as beginning the "slope toward the end". Such thinking results from the simplistic model of an empire's history as consisting of two segements: "growth" and "decline". In reality, the history of the Byzantine empire is a complex sequence of alternating growth and decline.

I'd say that the first high point of the empire was the end of Justinian's rule, when the borders of the Byzantine empire bore some resemblance to the old Roman empire. A large numbers of events weakened the empire from this high point. They include the loss of much of the Italy to the Lombards in the 6th century and the gradual loss of Levant, Mesopotamia and North Africa to Muslim Arabs starting from the rise of the Umayyad Caliphate in the 7th century.

The Emperor Basil II's rule marked a strengthening of the empire, when the First Bulgarian Empire was destroyed in 1014 and Kievan Rus' accepted Byzantine-style Christianity.

In 1054 the Byzantine Church formally split with the Church of Rome, after a long period of growing tensions. The cause of the tensions was a combination of political rivalry between the Byzantine Emperor and the Pope and religious divergence due to movements such as Monophysitism, Monothelitism and Iconoclasm within the Byzantine Empire. This eventually contributed to the demise of the Empire due to its religious and therefore political isolation. Territorial decline continued with the loss of of remaining Italy to the Normans in the 11th and 12th century and the gradual Seljuk takeover of Anatolia starting in the 11th century.

Another high point was the Komnenian restoration in the 12th century, during which much of Anatolia was temporarily recovered. After that, there was return to severe dynastic strife which was another major factor in Byzantine decline.

During the Fourth Crusade, Constantinople was taken by the (Roman Catholic) crusaders in 1203. The key role in this event belonged to the naval–commercial rivalry between the Byzantine Empire the Republic of Venice. The crusaders couldn't pay for the fleet Venice supplied them and agreed to attack Constantinople as a compensation. The fall of Constantinople caused the split of the Empire into three parts: Empire of Nicaea, Empire of Trebizond and the Despotate of Epirus. The crusaders created their own state: the Latin Empire.

The Empire of Nicaea under Michael VIII Palaiologos managed to restore the Byzantine Empire by taking back Constantinople in 1261. This caused the demise of the Latin Empire, in what is probably the last high point in Byzantine history. The rise of the Ottoman Empire in 1299 caused the gradual eclipse of the Byzantine, with Constantinople falling again in 1453 and remaining "mainstream" Byzantine territory in Morea (Peloponnese peninsula) falling in 1460. The Empire of Trebizond fell in 1461 and the Despotate of Epirus lingered on until 1479 when it also was taken over by the Ottoman Turks.