Even as Anselm was being invested as archbishop and it seemed he and William had reconciled their differences, William made a move that caused the first big clash. William's father, William the Conqueror, had left him England, but William senior's original possession of Normandy on the continent went to his elder son, Robert Curthose. William junior wanted to rule Normandy as well, so planned a takeover. Plans like this required soldiers and supplies, and those required money. The quickest way for a king to raise cash was to tell everyone to give you some.
So William sent to Anselm, asking for £1000. Anselm offered £500. William felt he was owed money for Anselm's new position (something called annates, which maybe we'll go into someday). Anselm decided to pursue his own agenda. He asked William to fill all the vacant church positions and allow Anselm to enforce canon law. William refused. Anselm withdrew any offer of funds, saying "that he [Anselm] disdained to purchase his master's favor as he would a horse or ass." William was said to reply that he didn't want Anselm's money or blessing for the endeavor, because "I hated him before, I hate him now, and shall hate him still more hereafter."
Anselm really wanted to make his appointment official by receiving a pallium from the pope; William had refused Anselm's travel for this purpose earlier. A meeting of nobles and bishops gathered to discuss this. William ordered the bishops not to treat Anselm as their archbishop, and they caved to the king. The nobles, however (many of whom did not approve of William's rule) supported Anselm. Secretly, Anselm asked two men to travel to Pope Urban II and request the pallium. They were Bishop of Exeter William of Warelwast and Archbishop Gerard of York.
They persuaded Urban to send a papal legate with the pallium. The legate met with the Bishop of Durham, who represented the king (and had argued against allowing Anselm to go get the pallium himself). William agreed that he would support Urban (over Antipope Clement III), in exchange for the right to block papal legates and intercept any papal letters to clerics. This was unacceptable, so William tried to sell the pallium to anyone who would take it and replace Anselm. No one would take it (or the price was too high). He tried to get money from Anselm for the pallium; Anselm refused. William then wanted to personally put the pallium on Anselm, but Anselm refused again: this act would suggest that the king had the authority of a pope over the archbishop.
Finally, the pallium was placed on the altar at Canterbury Cathedral, and on 10 June 1095 Anselm placed it on himself (seen above in a 20th century representation by E.M.Wilmot-Buxton).
A few months later, Urban would declare the First Crusade. William continued to deny Anselm's attempts at reform and church independence, and Anselm even had to go into exile. But it was around this time that he wrote the most consequential piece of Christian theology in the Middle Ages, an essay titled Cur Deus Homo.
Anselm had a better relationship with William's successor, his younger brother Henry. For that to happen, however, William had to die, and the circumstances of his death have inspired a conspiracy theory that has never died. I'll tell you that story next time.