A military order (militaris ordo) is a Christian religious society of knights. The original military orders were the Knights Templar, the Knights Hospitaller, the Order of Saint James, the Order of Calatrava, and the Teutonic Knights. They arose in the Middle Ages in association with the Crusades, both in the Holy Land, the Baltics, and the Iberian peninsula; their members being dedicated to the protection of pilgrims and Christians, as well as the defence of the Crusader states. They are the predecessors of chivalric orders.
Most members of military orders were laymen who took religious vows, such as of poverty, chastity, and obedience, according to monastic ideals. The orders owned houses called commanderies all across Europe and had a hierarchical structure of leadership with the grand master at the top.
The Knights Templar, the largest and most influential of the military orders, was suppressed in the early fourteenth century; only a handful of orders were established and recognized afterwards. However, some persisted longer in their original functions, such as the Sovereign Military Order of Malta and the Order of Saint John, the respective Catholic and Protestant successors of the Knights Hospitaller. Those military orders that survive today have evolved into purely honorific or ceremonial orders or else into charitable foundations.
In 1053, for the Battle of Civitate, the Knights of Saint Peter (Milites Sancti Petri) was founded as a militia by Pope Leo IX to counter the Normans.
In response to the Islamic conquests of the former Byzantine Empire, numerous Catholic military orders were set up following the First Crusade. The founding of such orders suited the Catholic church's plan of channeling the devotion of the European nobility toward achieving the Church's temporal goals, and it also complemented the Peace and Truce of God. The foundation of the Knights Templar in 1118 provided the first in a series of tightly organized military forces for the purpose of opposing Islamic conquests in the Holy Land and in the Iberian Peninsula — see the Reconquista — as well as Islamic invaders and pagan tribes in Eastern Europe which were perceived as threats to the Church's supremacy.