1070AD Seljuk Turks take Jerusalem & threaten all Christendom

From: http://en.wikisource.org/wiki/Catholic_Encyclopedia_(1913)/Crusades

I. ORIGIN OF THE CRUSADES

...Instead of diminishing, the enthusiasm of Western Christians for the pilgrimage to Jerusalem seemed rather to increase during the eleventh century. Not only princes, bishops, and knights, but even men and women of the humbler classes undertook the holy journey (Radulphus Glaber, IV, vi). Whole armies of pilgrims traversed Europe, and in the valley of the Danube hospices were established where they could replenish their provisions. In 1026 Richard, Abbot of Saint-Vannes, led 700 pilgrims into Palestine at the expense of Richard II, Duke of Normandy. In 1065 over 12,000 Germans who had crossed Europe under the command of Günther, Bishop of Bamberg, while on their way through Palestine had to seek shelter in a ruined fortress, where they defended themselves against a troop of Bedouins (Lambert of Hersfeld, in "Mon. Germ. Hist.: Script.", V, 168). Thus it is evident that at the close of the eleventh century the route to Palestine was familiar enough to Western Christians who looked upon the Holy Sepulchre as the most venerable of relics and were ready to brave any peril in order to visit it. The memory of Charlemagne's protectorate still lived, and a trace of it is to be found in the medieval legend of this emperor's journey to Palestine (Gaston Paris in "Romania", 1880, p. 23).

The rise of the Seljukian Turks, however, compromised the safety of pilgrims and even threatened the independence of the Byzantine Empire and of all Christendom. In 1070 Jerusalem was taken, and in 1091 Diogenes, the Greek emperor, was defeated and made captive at Mantzikert. Asia Minor and all of Syria became the prey of the Turks. Antioch succumbed in 1084, and by 1092 not one of the great metropolitan sees of Asia remained in the possession of the Christians. Although separated from the communion of Rome since the schism of Michael Cærularius (1054), the emperors of Constantinople implored the assistance of the popes; in 1073 letters were exchanged on the subject between Michael VII and Gregory VII. The pope seriously contemplated leading a force of 50,000 men to the East in order to re-establish Christian unity, repulse the Turks, and rescue the Holy Sepulchre. But the idea of the crusade constituted only a part of this magnificent plan. (The letters of Gregory VII are in P.L., CXLVIII, 300, 325, 329, 386; cf. Riant's critical discussion in Archives de l'Orient Latin, I, 56.) The conflict over the Investitures in 1076 compelled the pope to abandon his projects; the Emperors Nicephorus Botaniates and Alexius Comnenus were unfavourable to a religious union with Rome; finally war broke out between the Byzantine Empire and the Normans of the Two Sicilies....