In the 12th century the Franks and Roman Catholic crusades were going on to get back Jerusalem and the holy land from the muslims. The Franks and Roman Catholics believed using violence and force was the way to get back what was theirs. The crudsades were happening in both the Middle East and Western Euroupe.
Middle East-
The holy land, which is located in jerusalem, is in the middle east. It has both religious value to the Chrisitans and the Muslims. It is important to the Christians because it is where Jesus was born, lived, and was crucified, and resurected. It is important to muslims because it is said that Muhammed ascended into heaven. Muhammed is the prophet of God in the muslim relgion so it is one of the most religious/sacred sites in the Muslims relgion as well.
Western Euroupe-
The Christians were trying to get back the holy land and were saying it was "Just War" to attack the Muslims to get back the holy land. People believed that if you died in battle trying to get Jerusalem back you would be forgiven of your sins and would go straight to heaven. In 1603 Pope Alexander said that they go invade the Muslims to try to gain the holy land back.
Conclusion-
The main reason the crusades were going on was a fight for the holy land. It was sacred to both religions and they both wanted control of Jerusalem. Christians actually thought that killing Muslims to try to retatin the holy land was good and that they would be forgiven for their sins. The Pope even approved of the crusades. They believed it was just because it was such a sacred place in their relgious views.