Why did the Iroquois League switch sides during the French and Indian War?

During the French and Indian War, the Iroquois sided with the British against the French and their Algonquian allies who were traditional enemies of the Iroquois. The Iroquois hoped that aiding the British would bring favors after the war. A key figure was Sir William Johnson - an Irish immigrant with an estate in the New York colony. He had an excellent relationship with the Iroquois - especially the Mohawks.