The Crusades were a series of religious wars, blessed by the Catholic Church starting in the early 11th century. The main focus of the crusades was recapture of Christian access to the holy places in and near Jerusalem. The Crusades were launched in response leaders of the Byzantine Empire asking for aid in the fight against Turkish muslim expansion into Byzantium.
Soldiers became crusaders by taking vows; the Pope granted them plenary indulgence, the absolution of all their sins. "crusade" is derived from the French term for taking up the cross.
The term "crusade" is also used to describe religiously spurred campaigns conducted between 1100 and 1600 usually against pagans, heretics, and peoples under the ban of excommunication.
The Crusades had major far-reaching political, economic, and social impacts on western Europe, and as well ruined the Byzantine Empire.