In the winter of 1077, German King Henry IV trudged through a snowy mountain pass in the Italian Alps.
He was headed for the village of Canossa in northern Italy, on a mission to win forgiveness from Pope Gregory VII. A few months earlier, Gregory had excommunicated Henry over a power struggle between the two men. Henry had wanted to appoint his own local bishops. The pope refused to allow it.
When Henry — who would eventually be made the Holy Roman Emperor — arrived at Canossa, he stood outside the pope's castle for three days. On day three, the pope emerged and absolved Henry with a papal kiss.
Historian Tom Holland tells All Things Considered host Guy Raz that Henry's journey to Canossa was "an episode as fateful as any in Europe's history." He calls it Europe's first great revolution.
The story is the backdrop to Holland's new book The Forge of Christendom: The End of Days and the Epic Rise of the West. It's an account of Europe in the years before and after the turn of the first millennium.
Henry IV presumed that he had not only the right but also the duty to poke his nose in the affairs of the church, Holland says. Gregory's victory intended to assure that the business of the church was of the church alone.
Paradoxically, the incident, known as the "Investiture Crisis," eventually led to the idea of separation of church and state.
Holland says the period was a time of premillennial angst among Europe's religious figures and widespread fear of the end of days.
There were people in the 10th century who were bracing themselves for the worst, Holland says.
When the apocalypse failed to occur, Holland says, European clerics and petty aristocrats began to come to terms with the idea of building a "New Jerusalem" in Europe. It was from that idea that some of Europe's medieval institutions emerged.
For example, it was in this period that Fulk Nerra, "the black Count of Anjou" in France, pioneered the construction of castles, giving him unrivaled power. Nerra recruited local bodyguards — "mail-clad thugs," as Holland calls them — who would come to be known as "chevaliers," or knights.
It's a period that eventually led to the secularization of Europe — but not for another several bloody centuries.