As with most events in the Middle Ages, we don't know as much as we'd like to.

This "Peace League" was one episode in the long "Peace of God" movement which grew in the 10th and 11th centuries. The goal of this movement was, well... peace. There were many threats to peace at this time; Viking raids and conquests, the partition of the Carolingian Empire, the growing power of the Normans, civil wars, the Investiture Controversy... the list goes on. Medieval Europe was a violent and dangerous place, and there was external and internal pressure on the church to do something about it.

So local clergy attempted to reign in these troubles. There were generally two approaches:

A) Declare that there will now be peace, and apply political, spiritual, and economic leverage on troublemakers to restrain the violent

B) Declare that there will now be peace, and crush anyone who resisted

Most advocates for the movement chose option A. Take this proclamation by the archbishop of Arles from 1035:

In the name of God, the omnipotent Father, Son, and Holy Spirit. Reginbald, archbishop of Arles, with Benedict, bishop of Avignon, Nithard, bishop of Nice, the venerable abbot Odilo [of Cluny], and all the bishops, abbots, and other clergy of Gaul, to all the archbishops, bishops, and clergy of Italy, grace and peace from God, the omnipotent Father, who is, was, and shall be.

1. For the salvation of your souls, we beseech all you who fear God and believe in him and have been redeemed by his blood, to follow the footsteps of God, and to keep peace one with another, that you may obtain eternal peace and quiet with Him.

2. This is the peace or truce of God which we have received from heaven through the inspiration of God, and we beseech you to accept it and observe it even as we have done; namely, that all Christians, friends and enemies, neighbors and strangers, should keep true and lasting peace one with another from vespers on Wednesday to sunrise on Monday, so that during these four days and five nights, all persons may have peace, and, trusting in this peace, may go about their business without fear of their enemies.

3. All who keep the peace and truce of God shall be absolved of their sins by God, the omnipotent Father, and His Son Jesus Christ, and the Holy Spirit, and by St. Mary with the choir of virgins, and St. Michael with the choir of angels, and St. Peter with all the saints and all the faithful, now and forever.

4. Those who have promised to observe the truce and have wilfully violated it, shall be excommunicated by God the omnipotent Father, and His Son Jesus Christ, and the Holy Spirit, from the communion of all the saints of God, shall be accursed and despised here and in the future world, shall be damned with Dathan and Abiram and with Judas who betrayed his Lord, and shall be overwhelmed in the depths of hell, as was Pharaoh in the midst of the sea, unless they make such satisfaction as is described in the following:

[There is then a long list of spiritual and economic punishments for breakers of the truce depending on what they have done]

We have dozens of documents like this, and all are pretty much the same - on certain days there was to be no violence of any kind, and anyone who broke it would be excommunicated and prosecuted.

The archbishop of Bourges, Aimon, was not inclined to this method of peace. Our archbishop chose option B. The story comes to us through Andrew of Fluery's Miracles of St. Benedict:

At this very same time, archbishop Aimon of Bourges wished to impose peace in his diocese through the swearing of an oath. After he had summoned the fellow bishops of his province and had sought advice from these suffragens, he bound all men of fifteen years of age and over by the following law: that they would come forth with one heart as opponents of any violation of the oath they had sworn, that they would in no way withdraw secretly from the pact even if they should lose their property, and that, what is more, if necessity should demand it, they would go after those who had repudiated the oath with arms. Nor were ministers of the sacraments excepted, but they frequently took out banners from the sanctuary of the Lord and attacked the violators of the sworn peace with the rest of the crowd of laypeople (populus). In this way they many times routed the faithless and brought their castles down to the ground. With the help of God they so terrified the rebels that, as the coming of the faithful was proclaimed far and wide by rumor among the populace, the rebels scattered. Leaving the gates of their towns open, they sought safety in flight, harried by divinely-inspired terror. You would have seen [the faithful] raging against the multitude of those who ignore God, as if they were some other people of Israel. Presently they presently trampled [the rebels] underfoot so that they forced them to return to the laws of the pact which they had ignored.

So there was a local conflict involving several settlements, and Aimon decided he would bring the whole lot of them to heel, thereby creating and enforcing peace. They got off to a great start, enforcing peace throughout the land, but then something went wrong:

When by the will of God they had, trusting in the help of divine strength, established peace in every direction, ambition (the root and aid of all evil) began to seep along the stalks of such good works. They forgot that God is the strength and rampart of his people, and ascribed the power of God to their apostate power. . . Thus the aforementioned bishop was touched by the sting of mammon and raged around and around in blind ambition. Unmindful of his episcopal dignity, he attacked Beneciacum, the castle of one Steven, along with a multitude of the people (populus) of Bourges. He reproached Steven for the fault of having ignored the peace, he tried to burn the castle with flames and ordered it to be levelled to the ground, as if he were exacting the vengeance of God upon it. They burned the castle, which was hemmed in on all sides by the siege, with more than one thousand four hundred people of both sexes inside. Steven alone of that great number escaped, although his brothers, wife, and sons were all consumed by the fire, and he placed the laurel wreath of his great victory on their wretched heads. The inhabitants of that region for a radius of fourteen miles had fled to this castle and, since they feared the theft of their possessions, they had brought them along. The cruel victors were hardly moved by the laments of the dying, they did not take pity on women beating their breasts, the crowd of infants clinging to their mothers' breasts did not touch any vein of mercy. And so the just bore responsibility for the crime of the iniquitous and the just perished in place of the impious.

The enforcers of peace had become the warmongers. Naturally, this incurred the wrath of God, and also galvanised the warring peoples of the region against the archbishop under the leadership of Odo, a man of unknown origin. We are told that Aimon rocked up to the battle only to realise he was severely outnumbered by the coalition against him. Aimon was utterly crushed, and the river they fought next to was blocked by the bodies of horses and men.

That's about the sum total of what we know - an all too common story of someone with decent intentions being blinded by arrogance and ambition, with the people they swore to protect suffering the most for it. That it fits this morality tale is a tad suspect - Andrew of Fluery clearly has an agenda here, but as far as we know the story itself is a true one.