Some of us are going through Sketches from Church History by S M Houghton one small chapter at a time. (By the way, the book has pictures.) Aiding us in this study is the work book by Rebecca Frawley. Both are Banner of Truth books.
Now we are at
Chapter 12  The Crusades

Emperor Constantine and his mother Helena had built costly churches in Palestine, and so Palestine was regarded as the Holy Land and Jerusalem as the Holy City by Christians.

In 636 Jerusalem was captured by the Mohammedans under Caliph Omar. Except for some restrictions, by and large, Christian pilgrims were able to visit the “Holy Land.”
In the 11th century, Jerusalem was captured by fierce Seljuk Turks. After this, Christian pilgrims were subjected to harsh treatment. This led to the crusades.

The Crusade movement comprised a series of religiously-sanctioned military campaigns, which lasted two centuries and cost the lives of nearly five million Europeans, in addition to the lives of muslims and others lost. The Crusades are an unfortunate part of church history. The wars had no scriptural basis and were uncalled for. Christ’s kingdom is not of this world, and the warfare of Christians for religious purposes should only be spiritual. Having said this, we must remember that many of the crusaders were brave and imagined that they were fighting for a good cause, wanting to recapture what they believed was their Holy Land so that Christians may be able to worship there in peace.

Something about some of the crusades:
First Crusade: Preached by Peter the Hermit and Pope Urban II. The Pope promised forgiveness of sins to all crusaders. This is ofcourse totally unbiblical. Untrained people, even women and children, went and were defeated by the Turks in Nicea. A later group of men trained in warfare had success. They captured Jerusalem, carried out a terrible massacre of the inhabitants including women and children, and then held the city for 50 years.
Second Crusade: In 1147 Bernard of Clairvaux (who wrote hymns like ‘O Sacred Head once wounded’) undertook a crusade, which ended in failure. This encouraged Saladin to capture Jerusalem in 1187.
Third Crusade: This was undertaken by three European kings. Frederick Barbarossa, Emperor of Germany, Philip Augustus, King of France, and Richard Coeur-de-Lion, King of England. This ended in failure.
Children’s Crusade: This crusade came about because Stephen, a French boy, claimed to have seen a vision, and Nicolas, a German boy, spread the news in his country. Thousands of children joined this movement. Many died on the way in the snowy alps. The sea did not divide as they expected and many returned. Seven ships took 5000 children on board, to take them to Palestine. Two ships struck rocks and sank. The other five were slave ships and took the children to Palestine where they were sold as slaves, never to be heard of again.
Sixth Crusade: Promoted by St Louis, the French King ended in failure.
Seventh Crusade: By St Louis, joined by Price Edward I of England, also ended in failure.

I’d like to add a personal note here. The Crusades are a part of history that most of the world may have forgotten. But they are indoctrinated in the minds of Muslims world over in a highly imbalanced and subjective fashion. I was rather taken aback by the vehemence of a good Muslim friend over this subject. With Middle Easterners, history, although seldom objective, matters.
To read more about Sketches from Church History