The Crusades were a response to a call by Pope Urban after the Muslim conquest of Jerusalem in 1095.
First Crusade (1 1096 to 1099): In this, five armies of nobles and one army of peasants headed towards Jerusalem and managed to seize it. This crusade was partially at the request of the Byzantine emperor who was also being attacked by Turks.
The Second Crusade (1145–1149) was organized by Pope Eugene III. It was led by the French king Louis VII of France and the German king Conrad III. Although it did reach Jerusalem, it lost many other battles and is generally considered a failure.
The Third Crusade (1189–1192) was organized by the English King Henry II of England, the French king Philip II of France, Richard Lionheart and Holy Roman Emperor Frederick Barbarossa.
The Children’s Crusade was organized by Stephen of Cloyes in 1212. The 15,000 children disappeared on their way to Jerusalem.
The Fourth Crusade (1202–1204), called for by Pope Innocent III, invaded and conquered the Byzantine (Christian) city of Constantinople and did not continue on to Jerusalem.