Religious factors:
1. Seljuk Turks: In the 11th century, the Seljuk Turks, a Muslim dynasty, conquered vast territories in the Middle East, including Jerusalem and the Holy Land. This prevented Christian pilgrims from visiting the holy sites and threatened the Byzantine Empire, the main Christian power in the region.
2. Byzantine Emperor's Call for Help: The Byzantine Emperor Alexius I Comnenus appealed to the Western European Christians for military assistance against the Seljuk Turks. He promised rewards and spiritual benefits for those who participated in the holy expedition.
Political factors:
1. Power and Prestige: The Crusades provided an opportunity for European nobles, kings, and knights to gain power and prestige by participating in a holy war. They could acquire new territories and gain military glory by defeating the Muslims.
2. Unification of Christendom: The Crusades helped unite Christians across Europe under a common goal. It also diverted the focus of internal conflicts within Europe towards an external enemy.
Economic factors:
1. Trade and Commerce: The Crusades stimulated trade and commerce between Europe and the Middle East. The Crusaders needed supplies, weapons, and other goods, which created new markets and opportunities for European merchants.
2. Urban Growth and Feudal Economy: The Crusades had a significant impact on the urban centers of Europe, as they provided a market for goods and services. The wealthy feudal lords and clergy who participated in the Crusades often invested their wealth in the urban areas, leading to the growth of cities and towns.
3. Land Acquisition: Some Crusaders sought to acquire land and territories in the Holy Land, which was seen as a valuable possession due to its religious significance.
These religious, political, and economic factors combined to trigger the Crusades, leading to a series of military expeditions from Western Europe to the Middle East that aimed to recapture Jerusalem and the Holy Land from Muslim control.