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Ethelred the Pious
The late Eleventh Century was disastrous for the Eastern Roman Empire. The Seljuk Turks, who had built an enormous Middle Eastern empire, dealt a decisive defeat to Byzantine forces at Manzikert in 1071 and proceeded to occupy the whole of Asia minor, capturing Nicaea in 1077. Emperor Alexios came to the throne in 1081 and began the work of stabilizing the crumbling empire and recovering lost territory.
To this end, Alexios in the 1090s reached out the the newly crowned Western emperor, Guilhem (or William) II. The intent was not merely to get aid, but to increase good relations between East and West, something supported by both monarchs. Guilhem planned a great eastern campaign for the year 1096.
Guilhem began to raise an army, gathering forces outside Piacenza and contracting ships in Bari to ferry them across the Adriatic. He also ordered his feudal nobles in Burgundy and Provença to equip knights to join him. At this point things started to drift out of the emperor's control, as the long-simmering issue of relations between Pope and Emperor got in the way of his plans. The Popes had been more or less subservient to the emperors for decades, ever since the rise of the Burgundian dynasty and the consolidation of their empire in the West.
By this time, the papacy was under the firm control of the imperial government. But encouraged by a spirit of religious reform, Pope Stephen IX still dreamed nostalgically of an independent, universal papacy. He saw in Guilhem's campaign an opportunity to extend his influence over the Christians of northern Europe, who had grown very independent in this era of Caesaropapism in the West. Without asking the emperor's permission, Stephen sent emissaries northward to the feudal kingdoms of Neustria, Lotharingia, and Germany, hoping that a zealous military campaign under papal leadership would stir up enthusiasm for Rome.
Close to the empire's borders, the Pope's messengers found few takers. The knights in those regions were engaged in constant border fighting with imperial vassals and were not inclined to join him in a fight far away. But further north, they found a nobility eager for a way to express their piety through their main calling, warfare. Some armed bands began heading south toward Italy, while others began making their way eastward directly to Constantinople.