The Seljuk Turks advanced to the Holy Land and took control of Palestine. This threatened pilgrimages being made to Jerusalem. In fear of the Seljuks, crusades started to take place.
The Third Crusade
We'll keep some cities in Palestine, but we'll stop attacking other cities.
The First Crusade was in June 1098 in Antioch, Syria and they only won because a traitor let them through the gates. A year later the crusaders surrounded Jerusalem and slaughtered the Muslims and Jews once they got in.
King Richard I Of England
I will get back Jerusalem and defeat the Muslims
The Second Crusade was launched in response to the Muslims banding together and capturing Edessa. The armies sent by Germany were beaten. The France king sent 50,000 crusaders to Damascus, but the Muslims beat them back. This crusade ended up failing.
The Children's Crusade
The Muslims came together under the rule of sultan Salah al-Din and led a renewed fight against crusaders. They gained Jerusalem and shocked the Europeans and started the Third Crusade. It was ended when the sultan and King Richard I (leader of Third Crusade) signed a peace treaty
Christian pilgrims are now allowed in Jerusalem.
King Richard I, also known as Richard the Lionheart, was the leader of of the Third Crusade. In 1191 he conquered Acre and tried to get Jerusalem and he also made arrangements with Salah al-Din for a prisoner exchange, but he grew impatient and killed all his Muslim prisoners. A year later a signed a peace treaty which ended the Third Crusade.
The Children's Crusade was one of the later crusades. It was formed by thousands of peasant children in 1212 from France and Germany. Very few reached the Holy Land and European ports, but were sold into slavery by merchants and most of them disappeared without a trace.