The Crusades, Christendom’s attempts to win back the Holy Land and Jerusalem, began in 1095 with the First Crusade.  The Muslims had taken Jerusalem in 1076.

Pope Urban incited the Christians of Medieval Europe with the words: “Christians, hasten to help your brothers in the East, for they are being attacked. Arm for the rescue of Jerusalem under your captain Christ. Wear his cross as your badge. If you are killed your sins will be pardoned.”

The Crusaders had to follow an overland route to Constantinople, where they gathered in preparation for moving south to Palestine. By 1097, after a brutal journey to reach it, nearly 10,000 people had gathered in Constantinople.  http://www.historylearningsite.co.uk/cru2.htm

The Crusaders encountered little resistance for the most part, and reached Jerusalem by June 7, 1099.  They began their attack on the 13th, and by the 17th, had slaughtered every Muslim in the city.  They established the Kingdom of Jerusalem and held it for nearly 100 years.

By 1187, however, “Saladin had enough of broken treaties, renegade Crusader Princes’ (see Reginald of Chatillon) attacking Arab caravans and the harassment of his people. Saladin’s army started a march on Jerusalem. His army met up with King Guy [de Lusignon] at the Horns of Hattin on July 4th, 1187. Guy was poorly advised. He was greatly outnumbered but he attacked, and after a long, bloody battle, was taken prisoner. Balian of Ibelin was also captured at this time, but begged permission to return to Jerusalem to look after his ailing wife, the former wife of Amaury. Saladin wasn’t heartless, and allowed Balian to go.

Balian arrived in Jerusalem to find chaos. He placed himself in charge, as he was the highest ranking officer. He then proceeded to fortify the city in preparation for a possible siege. Saladin arrived at the Mount of Olives on September 26th. Balian held the city till September 30th, when he and Saladin finally agreed to come to terms and Balian surrendered the city to Saladin. The Crusaders left the city of Jerusalem, their capitol, to the armies of Saladin, thirty days later.”  http://www.medievalcrusades.com/kingsofjerusalem.htm

An associated aspect of the Crusades was the attack on European Jews along on the way.  One account of a massacre in Germany reads:  “As soon as the enemy came into the courtyard they found some of the very pious there with our brilliant master, Isaac ben Moses. He stretched out his neck, and his head they cut off first. The others, wrapped by their fringed praying­shawls, sat by themselves in the courtyard, eager to do the will of their Creator. They did not care to flee into the chamber to save themselves for this temporal life, but out of love they received upon themselves the sentence of God. The enemy showered stones and arrows upon them, but they did not care to flee, and [Esther 9:5] “with the stroke of the sword, and with slaughter, and destruction” the foe killed all of those whom they found there. When those in the chambers saw the deed of these righteous ones, how the enemy had already come upon them, they then cried out, all of them: “There is nothing better than for us to offer our lives as a sacrifice.” [The outnumbered Jews had no chance to win: Emico is reported to have had about 12,000 men.]

The women there girded their loins with strength and slew their sons and their daughters and then themselves. Many men, too, plucked up courage and killed their wives, their sons, their infants. The tender and delicate mother slaughtered the babe she had played with, all of them, men and women arose and slaughtered one another. The maidens and the young brides and grooms looked out of the Windows and in a loud voice cried: ‘Look and see, O our God, what we do for the sanctification of Thy great name in order not to exchange you for a hanged and crucified one….'”  http://www.fordham.edu/halsall/source/1096jews-mainz.html