Answer:
The main participants - the First Crusade were formed by many belligerent kingdoms. I believe that we can separate them into the Papal States, the Kingdom of France, the County of Flanders, and the Holy Roman Empire.
The primary reason - it was to free the Holy Land (Jerusalem), however, there were political and economical reasons behind this Pope decision.
Explanation:
The Kingdom of Jerusalem is the place where three faiths converge: Christianity, Judaism, and Islam. One of the Islamic precepts was jihad (striving), and according to the Islamic faith, Jerusalem should be conquested. In the 10th century, the Turks take control of the Holy Land, avoiding the pilgrim access to that place. In March 1095, the emperor of Constantinople asks for help, arguing that the pilgrims were suffering to reach Jerusalem. Pope Urban II decides to help and sends soldiers to retake the Holy Land. However, there were more than just faith and love for Christianity. To the Church, it was a great opportunity to create good relations with the Bizantine Orthodox Church, after the East-West Schism in 1054, when the Christian Church divided itself into two main powers, each one ruling a part of the world.
