Jerusalem had been under Muslim (those that follow Islam, a religion centered around the Quran) control since 637 CE, when it was absorbed into the growing Islamic Empire. After this time, Christian pilgrims and merchants had peacefully lived with Muslims in the region for centuries. Then, in the mid 11th century, things changed.
In 1050, a more aggressive Islamic group, the Seljuk Turks, took control of Jerusalem. This new Islamic regime threatened not only Christian traders and pilgrims in the region but also in the city of Constantinople, the capital of the Christian Byzantine Empire. This growing threat caused the Byzantine emperor to send a plea for support to Pope Urban II in 1096. The consequence of his response, was the start of the First Crusade.
The First Crusade had a very difficult journey getting to the Middle East. There were about 30,000 foot soldiers and 10,000 knights on horseback.
They could not use the Mediterranean Sea as the Crusaders did not control the ports on the coast of the Middle East. Therefore, they had to cross land. They travelled from France through Italy, then Eastern Europe and then through what is now Turkey. They covered hundreds of miles, through scorching heat and also deep snow in the mountain passes.
The Crusaders ran out of fresh water and food was bought from local people but at very expensive prices.
Disease was common especially as men were weakened by the journey and drinking dirty water. Dysentery (an infection of the intestines) was common. Heat stroke also weakened many Crusaders. Disease and fatigue affected rich and poor alike.
By 1097, nearly 10,000 people had gathered at Constantinople ready for the journey to the Holy Land.
The first target of the Crusaders was the important fortress city of Nicaea. This city was taken by the Crusaders without too much trouble as the man in charge of it was away fighting.
The next target for the Crusaders was Antioch - a strongly protected Turkish city.
They were four separate Crusader armies in the First Crusade but also a large number of smaller armies. However, there was no command structure which led to problems with communications. It took a seven month siege (a military operation in which the enemy surrounds an area, cutting off essential supplies with the hope of a surrender) before the city fell.
The next target was Jerusalem.
The attack and capture of Jerusalem started in the summer of 1099. Jerusalem was well defended with high walls around it. The first attacks on the city were not successful as the Crusaders were short of materials.
Once the Crusaders had managed to get over the walls of Jerusalem, the Muslim defenders there ran away.
The Crusaders "killed whoever they wished". Those Muslims who had their lives spared, had to go round and collect the bodies before dumping them outside of the city. The Muslims claimed afterwards that 70,000 people were killed and that the Crusaders took whatever treasure they could from the Dome of the Rock.
The Crusaders took Jerusalem in 1099 and created the Kingdom of Jerusalem. The Crusaders held the city and the land around it for the next 87 years.