The Second Crusade
After the First Crusade in 1096 AD set up Christian kingdoms all along the coast of Israel and Lebanon, of course the Fatimid caliphs who had ruled that area before were very upset. By 1144, a Mamluk general, Imad-ed-din Zangi, had managed to unite enough Turks and Arabs in his army to attack the Christian kingdoms. Zangi did not take Jerusalem, but he did take the Syrian city of Edessa nearby.
In Europe, people were very upset to learn that the Turks had taken Edessa. First, this was an Islamic defeat of Christianity. Second, it threatened the kingdom that the Normans of the First Crusade had set up, which was making them (and their European relatives) a lot of money.
The Pope ordered Bernard of Clairvaux (in France) to preach a second crusade to take it back and defeat Zangi. The young king of France, Louis VII, agreed to go, along with the queen, Eleanor of Aquitaine. So did Conrad III of Germany, the Holy Roman Emperor. Louis was 23 years old and Eleanor was 22. Conrad was 51 years old.
From beginning to end, though, this crusade was not successful. Most of Conrad's soldiers were killed by Seljuk Turks as they marched through Turkey. When Louis and Conrad reached Jerusalem, they decided to attack Damascus, which would have made up for the loss of Edessa. But their attack on Damascus failed, and the kings and queens went home in disgust. This would prove to be a model for the rest of the Crusades too: none of them were ever as successful as the First Crusade.
More about the Third Crusade
The Holy Roman Emperors (Germany)
The Capetians in France