Each of three major religious groups claimed Jerusalem in the land of Palestine as their holy city.
To Christians, it was the place where Jesus was crucified and ascended to heaven
To Muslims, it was the place where Muhammad met three earlier prophets Abraham, Moses and Jesus.
To Jews, it was the site of the ancient temple built by Solomon.
In 600 CE, Arabs conquered the city.
The Arabs continued to allow Christian and Jewish pilgrims to visit Jerusalem. They even allowed Jews and Christians to live in the city.
In 1095, a different tribe of Arabs took over Jerusalem. They refused to allow Jewish and Christian pilgrims to enter the city.
The Pope heard about this and called for Christians to do something. He asked for a volunteer army to go and take back Jerusalem. About 30,000 knights and other fighting men took up the challenge and left to retake Jerusalem. As a sign of their fighting for their religion, each crusader wore a red cross on their shield, armor, helmet, cape or other clothes.
After two years of travel, hunger, disease, and even fighting among themselves, the Crusaders reached Palestine. After some very hard fighting and a three month siege of the city, the Crusaders won and recaptured Jerusalem. Some of the knights stayed to defend the city, but most returned to their homes. One of the unexpected results of the Crusades was that the Crusaders brought back new foods, literature, art, inventions, and ideas from the Arabs.
Success was short lived. About 50 years later, Arabs again captured Jerusalem. The new pope called for another crusade to retake the city. There were several more crusades none of which were successful.
There was an even greater tragedy in the making. One of the Crusades was the Children's Crusade. A child in France had a vision (supposedly a message from god) and his belief was that only children were pure enough to be allowed to capture the holy city. Soon thousands of children from France and Germany gathered to try and reach Palestine. Thousands died of hunger, freezing to death, disease, and accidents before they even reached the Mediterranean Sea. When they finally got to the sea, there was no way to get to Jerusalem. The children who survived went back home in disappointment. Europeans never gave up the idea of recapturing Jerusalem, they just sort of stopped trying.