The Peace and Truce of God was an anti-war movement that began at the end of the tenth century, and (according to Larry Siedentop) was the first such mass movement in recorded history.

From Siedentop’s Inventing the Individual: The Origins of Western Liberalism:

In 975 the Bishop of Le Puy convened a meeting of the knights and peasants of his diocese, eliciting from them an oath to respect the property both of the church and of the paupers or the ‘powerless’. In 989 a church council in Burgundy went even further. It excommunicated ‘those who attacked bishop, priest, deacon or clerk, while at home or travelling; those who robbed a church; those who stole a beast from the poor or the tillers of soil.’ [1] By the end of the century many other public meetings and church councils had extended the ‘Peace of God’, so that it included ‘pilgrims, women and children, labourers and instruments of their work, monasteries and cemeteries’. These were to be left ‘undisturbed and in perpetual peace’.

Such councils had first appeared in the south of France. But they soon spread to its northern regions as well. Indeed, the movement became an irresistibly popular one. ‘Peasants of every class, from the most prosperous, through the middling ranks, to the lowest of all’ flocked to the councils. The power of the movement was such that by 1017 it constrained the nobles and knights to accept a ‘truce of God’. They swore ‘to desist from all private warfare from noon on Saturday until prime on Monday’.

‘This would allow due reverence to be paid on the Lord’s Day; those who broke this ‘truce of God’ would be cut off from the sacraments of the church and the society of the faithful in life; no priest might bury them, no man might pray for their soul. Those who swore to observe the truce were assured of absolution from God…’ [2]

[1] Deanesly, A History of the Medieval Church 590-1500, p98

[2] James, The Origins of France, p207

Worth noting that Siedentop is writing about this from the perspective of the history of ideas.