In 1095 C.E., at the Council of Clermont, Pope Urban II gave a speech that caused an uproar to the whole of Europe, He called on every Christian to a great journey to retake the Holy land that was conquered by the Muslims 300 years ago. 

The Church has never seemed concerned about retaking the holy land before, and the pilgrimages to Jerusalem continued without disturbance through the past years, the pope was obviously not only for retaking the holy land. It was the Byzantine Empire that was asking for help from the pope. The Muslim Turks were conquering nearly every land of Anatolia so the emperor wanted the pope to send an army and give help so that he could retake the land that was under Byzantium's control before, the pope on the other hand had another idea, in fact the Pope was having a war with the holy roman empire, this story starts with the previous pope Gregory VII. In 1077 C.E., the Holy Roman Emperor Henry IV  was ex-communicated by Pope Gregory VII, he had to stand in the cold winter and beg the Pope for pardon so he could restore his identity, this event is called the Road to Canossa. After he restored the identity of a Christian, he started to suppress the pope for 7 years before he died in Italy, the Pope after Gregory VII, Urban II was also suppressed, he couldn't even enter Rome. So Urban II wanted to use this Crusade and regain the power he should've had. After the speech, Peter the Hermit was the first to call for a crusade, but the people who joined him were mostly peasants, even though it also answered the call of Pope Urban II, thousands of people joined this Crusade, Even though this army seemed to be powerful and huge, it only entered Asia Minor before the Seljuk Turks defeated it. Along the way, the army didn't have enough forage to support it, so the army plundered the cities and villages to get enough forage. 

After the speech at Clermont, preparations for the Crusade started in both the East and West. The main Crusade force(not the People's Crusade) consisted of four different contingents. The first army was led by Godfrey of Bouillon, he was the only major prince who was from the Holy Roman Empire, this force was also joined by his brother Baldwin. The second army was organized by Bohemond, who was a Norman from south Italy, he had fought with the Byzantium empire years ago but he was determined to come to profitable terms with his former enemy. The third force was led by Raymond of Saint-Gilles, it was also the largest army, Raymond himself was also the oldest of the princes, he was already 55 years old and he aspired and perhaps expected to become the leader of the entire First Crusade, meanwhile the Fourth Army was under the lead of Robert of Flanders. No king took part in the First Crusade, and most of them spoke French so they were also known by the Muslims as Franks. They assembled at Constantinople and were forced to sign an agreement where every land they would conquer in the east would be owned by the Byzantine Empire. Finally, late in May 1097, the Crusaders started to march toward Jerusalem, and after two years of fighting, they reached the Holy land, Jerusalem on June 7th, 1099, the siege only continued for 8 days until the Christian army finally accomplished their goal after years of expedition, to retake the holy land Jerusalem.