Still considered by many the eastern Roman Empire, Byzantium fell into renewed chaos in the second millennium after centuries of rule reestablished by military strength of Justinian, Maurice, and Heraclitus. Justinian had pushed the empire to its zenith in the sixth century, and other great emperors worked to hold onto its expansive territory. However, the cost in manpower and resources gradually weakened the empire as Arab strength grew. The Macedonian dynasty of Basil the first restored much of the declining Byzantine strength, but the death of Empress Theodora, childless at 76, left the empire without clear leadership in 1056. Her successor Michael VI abdicated to become a monk, and his successor Isaac I abdicated after nearly being struck by lightning, leaving rule to the wealthy Doukas family. They bloated the bureaucracy with highly paid but ineffectual leadership, undercutting the soldiers, who began to rebel on the frontiers.
As Nikephoros III Botaneiates, he attempted to establish a new court, but his efforts only worsened the confusion. The established bureaucracy became alienated and even more ineffective while Botaneiates' co-emperor John Doukas and the old court began plots to overthrow Botaneiates. They concentrated their efforts on Alexios, who had continued to serve as a heroic general in the West and prepared to battle against invading Normans who fought to return the rule of the deposed Michael VII. Empress Maria of Alania, former wife of Michael VII and then wife of Botaneiates, adopted Alexios as her son and sent him to raise an army along with his natural and adoptive brothers. His mother, Anna Dalassena, escaped the suspicious palace guard and sought sanctuary at Hagia Sofia. The guards attempted to bring her home, but she exclaimed falsehoods of a plan to blind Alexios and his brother, whom she said had fled the city so that they might continue to serve the emperor. Although they tried to quiet her, she swore that she would only leave the church if Botaneiates gave his cross to her along with the vow that he would do no harm to her family.
Botaneiates became suspicious of her theatrical appeal and refused to give such a vow. He sent agents to find Alexios and his brother, who were indeed raising an army. They were brought back to Constantinople on April 1, imprisoned, and executed. Anna Dalassena hid in Hagia Sofia, which Botaneiates surrounded in a "siege" that prevented food other than sacrament to enter. Embarrassed, she was forced to leave the church and resigned to the convent of Petrion. Botaneiates set about rooting out the rest of the conspirators, which crippled the government in a crucial time.
The Normans under Duke Robert Guiscard continued their invasion of Byzantine lands after securing Sicily and Malta from the Muslim forces to the south. Using the political instability as a pretense, his forces conquered southern Italy and began an invasion of the Balkans with papal blessing. His army overwhelmed Botaneiates' defenses at Dyrrachium and moved toward Constantinople. Botaneiates attempted to defeat the army in the field, but his armies were repeatedly crushed, and the loot won by the Normans kept dissension at a minimum. Finally, in 1085, Robert sacked Constantinople and ended the Byzantine Empire.
Robert died after a few years' rule in Constantinople, and the Norman kingdom there collapsed under Seljuk attack. The ruling Seljuk emperor, Alp Arslan, had established a frontier of feudal "beyliks" (states) after defeating the Byzantines in Anatolia in 1071 at Manzikert. When the Seljuks splintered after the death of Malik Shah, Kilij Arslan founded the Sultanate of Rum in Asia Minor, pushing westward with the Emir Chaka of Smyrna until the Normans retreated back to Italy and Sicily. Muslim control rolled westward across the Balkans, butting up against Christendom's strongest center in Italy. Many talked of a united Christian force to drive back the Turks, but the most that Pope Urban II was able to manage was a bolstering of defenses for Italy and a push to retake lands along the North African coast to affirm Spain's Reconquista.
Meanwhile, trade flourished between the Italian city-states, such as Venice. With the Byzantine stranglehold on east-west trade removed, the Muslims gained great influence shipping good westward. Trade with Kiev at the north of the Black Sea brought Islam to Russia, where it made great advances overriding the Orthodox Christian beliefs adopted in the century before. Constantinople continued being one of the main hubs of the world, and Europe continued as a rich market for Islamic traders for centuries to come. Christian kingdoms, meanwhile, expanded southward and across the Atlantic Ocean to the New World there. While Europe underwent a Renaissance in the seventeenth century, many great minds traveled to the libraries of Constantinople to study, keeping the Islamic world apace with innovations in medicine, mathematics, and science.
In reality, Alexios successfully overthrew Nikephoros III Botaneiates. Having taken the vow that he could do no harm to the family and facing Alexios' army that bribed the city guard to enter Constantinople without a fight, Nikephoros had no choice but to abdicate. Alexios began the Komnenian dynasty, which revitalized the empire for a time. Perhaps most notable for history, Alexios pleaded for aid from Urban II in fighting the Seljuk Turks as Christian allies, which culminated in the Crusades.