The Byzantine history after Basileios II is primarily a rapid succession of coups and counter-coups. Two (partly) external factors would first seriously weaken the Middle Byzantine period and then end it: the arrival of the Turks and the fourth crusade.
The arrival of the Turks
The flourishing of the mid-Byzantine period was shattered when Turkish tribes led by Alp Arslan planned to move from the Russian steppes to Arabia via Armenia. Emperor Romanos Digenis encountered them in 1071 with a large but poorly organized army. They met at the town of Mantsikert, near Lake Van in Armenia. The Byzantine troops were devastatingly defeated and Emperor Romanos himself taken prisoner. Romanos managed to obtain very favorable conditions for his release with Alp Arslan, in fact nothing more than a tribute in gold. On his return to the capital, however, Romanos was captured and blinded by burning his eyes with a red-hot piece of iron. Subsequently, the new emperor refused to honor the agreements with Alp Arslan, even though they no longer had the troops to face Alp Arslan. The Turks responded by abandoning their march towards Syria and instead marching into Asia Minor. In a few years, almost the entire Asia Minor area of Byzantium was overrun. And although especially the dynasty of the Komnenes, from Alexios I Komnenos to Andronikos I Komnenos (1081-1180) managed to achieve a certain restoration, the Turks would never leave the area.
The Fourth Crusade (1202-1205)
The famous call of Pope Urban II of Clermont at the Council of Clermont on November 27, 1095 to start the holy war against the infidels and to free Jerusalem and the holy places from the hands of the Arabs, has led to a long series of conflicts between the Arabs and knights, monks, but also ordinary citizens from the Catholic, “Latin” West.
The Fourth Crusade made an effort to avoid the difficult overland road. Proclaimed by Pope Innocent III, the crusaders, led by Boniface of Montserrat, gathered in Venice and, from there, sailed to Egypt. Not having the necessary money to pay the Venetian fleet for the crossing, the Crusaders concluded an agreement with Venice to sail first to the Dalmatian coast, to visit the city of Zara, which had rebelled against Venetian rule and defected to the Hungarians, to be conquered for the Venetians. Thus, although the inhabitants of Zara hung crosses on the wall to underline their Christian identity, the city was captured in 1202 and handed over to Venice.
Meanwhile, Alexios Angelos, the son of a deposed emperor of Byzantium, had enlisted the help of Pope Innocent III and some European princes to be installed as emperor of Byzantium. He promised the Crusaders enormous sums of gold, and submission to the Pope in Rome if they helped him to the throne, after which the Crusaders accepted and Alexios joined the Crusaders in 1203. Alexios’ plan worked, in a moment! When the crusaders appeared before the walls of Byzantium, the reigning emperor fled and Alexios was installed as Alexios IV along with his father Isaac II as emperor. However, there turned out not to be enough money to pay the Crusaders, while the proposal to submit to the Pope sparked riots, in which Alexios IV was killed and his father imprisoned. A raid by the new emperor Alexios V on the crusaders turned out to be disastrous: after several days of fighting, the crusaders managed to invade Constantinople, followed by days of looting of the city, in which numerous coastal works were destroyed or transported to Venice. After this sack, Boudewijn of Flanders was inaugurated in 1204 as emperor of the now “Latin” empire. In the wake of the Fourth Crusade, countless knights from Flanders, France and Italy would now migrate to Greece to claim territories for themselves.