After the defeat at Manzikert, the Byzantine army was in tatters. The Emperor Romanus Diogenes had died in captivity, and his replacement Michael IV was incompetent. Also from this period begins increasing Western intrusion. A Norman adventurer Roussel was allowed into Anatolia as a mercenary with a force of Norman and Frankish knights to fight off Turcoman marauders. Instead, he set up his own short-lived state, harassing the Byzantines. At the same time, Byzantium was casting about for help. Preferential trade agreements were made with Venice, while the Empire itself was racked by another series of internal conflicts as claimants struggled for the throne. Ultimately the young general Alexius Comnenus took power in 1081, and spent the next years fighting off Norman incursions into Macedonia and Thrace under their leader Bohemond. In the early 1090s, he sent word to the Pope Urban II asking for some auxiliary forces to fight off the Seljuk Turks in eastern and central Anatolia, just as Michael IV had sent a similar request to Gregory VII in the 1070s. What he got was the relatively unruly First Crusade, none of whose warriors saw themselves as fighting to restore lands to the Byzantine state. Still, during the First Crusade, Alexius was able to restore some lands around western and southern Anatolia. Life was generally hard though for the average Byzantine. Nobles kept on usurping lands, or were granted hereditary lands through Pronoia grants in return for tax revenue and provision of military forces. Military recruiters scoured the countryside for soldiers, further depleting the agricultural base of the empire. Additional soldiers were acquired by purchasing the services of Hungarian, German, Anglosaxon, and Russian mercenaries. As well, the Byzantine government cut the central payroll, farming out much of the bureaucracy. When he died in 1118, Alexius had provided stability, but glory was a thing of the past.
John ruled from 1118-1143. There were sporadic problems with the Normans invading Albania, and increasing commercial privileges were given to the Latin, Italian states. Venice in particular was enlisted to fight Normans. These Italian merchants were becoming essential for Byzantium's economic survival, the Greeks resented them. While John had tried to cancel their privileges, he was forced to back down when Venetian ships plundered the Byzantine coast. Manuel Comnenus (1143-1180) was able to arrest their leaders, ending the Venetians privileges, yet Byzantium needed the commodities, and the Emperor found he had to grant similar dispensations to the Pisans and Genoans. He also began inviting larger numbers of Latin, French aristocrats to settle in the Empire, giving them Anatolian and Thracian lands.
Manuel was somewhat of a tragic figure. On the one hand he was able to establish a suzerainty over Crusader states from the 1140s, taking back more lands around Antioch for the Empire. He was thus the last Byzantine Emperor to really insist on a separate, non-dependent Imperial status vis-a-vis the West. Unfortunately, it all fell apart in the 1170s. In 1176, Manuel lead one of his many expeditions to central Anatolia. This time he was resoundingly defeated at Myriocephalum by Kilij Arslan, Sultan of the Seljuks of Rum. Though Manuel was allowed to retreat, this signifies the final breaking of Byzantium militarily. The army was destroyed and never fully replaced. Manuel died in 1180, and was succeeded by a regency headed by Maria of Antioch. She relied on the Italian merchants and French aristocrats settled in Constantinople. She was overthrown in 1182 by Andronicus, and the capital was rocked by riots in which Greeks massacred Latin elites and merchants. Andronicus' reign was a disaster. In 1184 the Normans invaded western Greece, taking Thessalonica. Andronicus then executed his generals, after which the urban mob rose, murdering him. At the same time, Serb tribes rose in the Balkans. Upon Andronicus' death, the Bulgars rose yet again, reestablishing their old kingdom. In order to fight them Isaac Angelos (1185-1195) relied on imperial warlords, who encroached on the Emperor's power and on state finances even more. In 1195, Isaac was overthrown by his brother Alexius III. Hoping he would shore up support for his position, he offered to support German Henry VI's impending Crusade with financial and military contributions. His inability to do so combined with Western designs on Byzantium and internal court intrigue to unseat him and pave the way for the Latin sack of Constantinople in the Fourth Crusade from 1202-1204. Byzantine rulers were then ejected from their own capital. Baldwin of Flanders became 'Latin Emperor' of Caonstantinople, and was hated by the populace, as a Barbarian usurper. Only in 1261, when Latin defenses were quite diminished and few in the West were concerned with the 'Latin empire', was Michael VII Palaeologus able to reenter the Byzantine capital, after forces under him had captured the city, massacred some Latins, and burned the Venetian quarter to the ground, forcing them to return to Italy.
What stands out most here is the complete disintegration of Byzantine state and society. Good reasons are yet to be provided. Part of it, however, involves the unwillingness of Byzantine elites to unite in support of single emperors, as well as their reluctance to give up any of their prerogatives as holders of lands that oppressed peasants and denied to the state the demographic background of successful Theme armies. Of course, by the 1100s, this may have been besides he point. The magnitude and speed of Turkic infiltration into Asia Minor by the 1110s--as far west as the Western Anatolian coastal mountains, in spite of Crusades--may have meant that the human and material basis for Byzantine survival was ebbing away. Increasingly, Byzantine aristocrats were reduced to bargaining for power and alliances with Turkic tribal leaders of the Seljuk state. Indeed, in typically prudent Byzantine fashion, Manuel was able to secure peaceful relations with the Seljuks through treaties in the 1150s and 1160s. Beyond this, though, the process of "De-Hellenization" and "Islamization" of Asia Minor was proceeding, visible in the decomposition of bishoprics, the decay of monastic life's material and human foundations. As well, the gradual entry into the region of the institutions of Islam, its clerics and preachers meant that eventually, conversion to Islam was making the region part of the Muslim homeland. Byzantium in quite a short time was becoming less Greek, more Muslim, and more riven by the tensions of a large foreign mercenary army and increasing Latin encroachment.
That is the last point of significance in this era. Greek Byzantines had always felt culturally, politically, and even religiously superior to the West, whose kings and even clerics were the descendents of the barbarian tribes who wrecked the Roman society that Byzantium was preserving. For their part the Latins envied the material and intellectual wealth of the East, yet never trusted Byzantium. The political culture was much more subtle, and Imperial willingness to engage in negotiations with the Muslims, or to ally with them if it served an Emperor's interests, almost smacked of infidelity to the Cross. Indeed, no emperor had kindly received Crusading armies, having viewed them as a nuisance at best, and a dangerous horde at worst. Basically, the Western attitude was that the Byzantines were too sophisticated, to subtle, not sufficiently honest, always holding back, and perhaps in secret league with the Muslims. Beyond this, they wanted a piece of the legendary Byzantine wealth. Greeks, then, especially after 1071, resented the fact that they--the protectors of true civilization, Romnanness, and proper Christianity--had been forced to fight the long fight against Islam and to come to terms with it, while the West was imposing its armies, ideas, and now economic strangulation on a glorious culture. This emotional-psychological background would be of great import in 1202-1204.
The details of the Fourth Crusade are quite confusing. Basically, Venice had offered to transport the crusaders to Palestine in return for large sums of money. Crusader leaders, however had been overly enthusiastic, and had exaggerated the numbers of warriors, thus purchasingg an excess of provisions and hull-space. They could not therefore pay Venice, so the Doge, Enrico Dandolo, proposed that the Fourth Crusade begin not by going to Palestine, but by sacking the Christian town of Zara on the Adriatic coast, which had recently thrown off Venetian domination. This they did, to the astonishment of Innocent III, the titular leader of the Crusades. He excommunicated the entire Crusade, relenting when they promised to actually go to Palestine. It was not to be, however. Isaac Angelus' son Alexius Angelus had escaped from Byzantine gaol after his father's overthrow and blinding. During the Zara sacking, he showed up a the court of Philip of Swabia--Isaac's son-in-law and the brother of German Emperor Henry VI. Alexius claimed that if the crusaders aided him in retaking the throne, he would reimburse the Venetians fully, provide troops and provisions to the Palestine-bound armies, and--impossibly--heal the 1054 Schism between the Eastern and Western Church. In short, he promised all when he had absolutely nothing. By June 1203, a Latin siege and penetration of Constantinople had landed Alexius on the throne, but he was understandably quite unpopular, and did not seem intent on repaying his debts. Crusaders were kept at a cold distance, and started to clash with the Greeks--Latins and Greeks had nothing but contempt for each other, built up over centuries of mutual distrust and cultural divergence. By 1204 the Crusader host--made up of professional sackers and plunderers--came to judge the Emperor and the Greek masses as perfidious, untrue to oaths (a cardinal sin in feudal terms), perhaps heretical, and in league with the Muslims by not providing what Alexius promised. They thus broke into the city, looting, burning, and pillaging. The Crusade, which was meant to avenge the losses of Hattin and restore Jerusalem to the Christians, ended by sacking the oldest Christian city in the world, and making the East-West rift permanent. The Fourth Crusade represented the total bankruptcy of the Crusading idea.