When Henry IV was excommunicated by pope Gregory VII in 1076, he was initially confident that his bishops would stick with him and that he could bring an army down to Rome to depose the pope. I mean his father had deposed 3 popes and up until recently the appointment of popes was very much an imperial prerogative.
But within just 8 months the support from the bishops crumbled away. There are many solid political reasons such as the bishops being afraid of their urban populations and cathedral chapters siding with the pope. But one specific event turned the tide decisively.
Bishop William of Utrecht, Henry IV. greatest cheerleader had been hurling insults and accusations of lewd behaviour at pope Gregory VII from the chancel of his church. He declared the King’s excommunication null and void and excommunicated the pope in turn.
That same day the cathedral was struck by lightening and the episcopal palace burnt down. And a few days later William had to take to his bed. He had suddenly become terribly ill and succumbed so quickly he could not even make confession and receive the last rites. The abbot of Cluny reported later that bishop William had appeared to him in a dream and had said that he was now suffering in the deepest recesses of hell.
When the magnates met for a Reichstag in Trebur in October 1076, many of the bishops had gone over to the opposite side giving the king an ultimatum that he would be deposed unless he gets released from the anathema before early February 1077.
Henry had to cross the alps in the midst of the coldest winter in living memory and beg for mercy from Pope Gregory VII at Canossa, one of the most famous events of the European Middle Ages.
If you want to hear the whole story, check out the History of the Germans Podcast available on my website www.historyofthegermans.com or on Apple Podcasts, Spotify or wherever you get your podcasts (link here:https://history-of-the-germans.captivate.fm/listen)