The Crusades involved multiple sides, primarily the Christian forces from Europe and the Muslim forces in the Middle East. The main Christian side consisted of various European kingdoms and principalities, including the Holy Roman Empire, the Kingdom of France, the Kingdom of England, and the Byzantine Empire. These forces were often united under the banner of the Catholic Church, which called for the liberation of Jerusalem and other holy sites from Muslim control.
On the Muslim side, the primary forces were the Seljuk Turks, the Ayyubid Sultanate, and later the Mamluk Sultanate. These Muslim forces were defending their territories and holy sites against the invading Christian armies.
Additionally, there were other groups involved, such as the Knights Templar and the Knights Hospitaller, who were military orders fighting on the Christian side. There were also occasional alliances and conflicts with local Christian communities in the Middle East, such as the Armenians and the Maronites.
In summary, the Crusades were a complex series of conflicts with multiple sides, primarily divided between Christian and Muslim forces, but also involving various other groups and factions.