(Latin: "Peace of God")
The Pax Dei was a popular religious movement of the Middle Ages. Beginning some time around 980, in the south of France, it was a peace movement dedicated to settling conflict and ameliorating the effects of war upon the innocent and weak. The movement was strongly millennial in its nature, and after its initial flowering in the late 10th and early 11th centuries, it slowly ebbed out, disappearing some time in the 13th century.
The focus of the Pax Dei was in the peace assemblies, mass congregations of people, who gathered in a troublespot to swear oaths of peace and to pray. Such meetings, exemplified by the Peace of Meuse in 1024 (where Robert II and the emperor Henry II met to swear peace), were often the occasion for the making of international peace treaties.
In one sense, the Pax Dei movement may be viewed as a sort of internal Crusade within Christendom, since one of its fundamental principles, enunciated at the Peace of Narbonne in 1054, was that shedding Christian blood was fundamentally the same as shedding Christ's blood (note that no such prohibition applied to shedding the blood of non-Christians).
Initially, the movement only tried to protect "the meek" (i.e. noncombatants). A similar movement, the Treuga Dei, sought to ban conflict on holy days. Related concepts that they were, Pax Dei and Treuga Dei soon merged into a single movement.
The climax of the peace movements was the declaration by Pope Urban II of a universal peace throughout Christendom, for the duration of what was later to be known as the First Crusade. The net effect of this was the aggression within the military class of Europe which was suppressed by the peace movements was channelled into violent action abroad, as Church-sanctioned crusaders.