The First Crusade was initiated by Pope Urban II, in 1095 the pope travelled to Clermont, Central France, to unite all Christians against the Muslims. In his speech he told Christians that they will be cleansed of all their sins if they die fighting for what the Muslims had taken away from them – Jerusalem.
The Siege of Nicaea was the first conflict between the Christians and Muslims. The siege went on for over a month before the city was conquered. Then the Crusaders split into two large armies and marched to Dorylaeum, but en route they were ambushed by Turks, 4000 Christians were believed to be killed. Nevertheless they regrouped and managed to hold their ground until Crusader reinforcements arrived. Next stop was Antioch.
Only half of the Crusaders that left Europe made it to Antioch, many had deserted but the majority were killed in battle or died from hunger and thirst. The Siege of Antioch lasted 8 months, and when the Christians finally captured the mighty city they heard news of a Muslim army that were on their way. The Muslim army was led by Kerbogha and when he arrived with his army they besieged the Crusaders. All was looking grim for the Crusaders but with the help of the Holy Lance they once again fought off the Muslims.
On Tuesday 6 June 1099, after 3 years and 2000 miles the Crusaders finally arrived at Jerusalem. Throughout their travels 90% of the Crusaders that left Western Europe had deserted or were killed, that left a shocking 10% of men to lay siege on the city which had heavy fortifications. After a six week siege in Jerusalem the Crusaders finally liberated the Holy city. What followed was a massacre of Muslims, Jews, and Pagans all brutally murdered at the hands of the Crusaders.
The First Crusade was a success, and with the creation of the Crusader states, the holy sites of Christianity were back in the hands of Christians.
Source – BBC The Crusades, Link