The spiritual union of the nations began before European contact. The visionary Hiawatha brought a message of peace to the original five nations. These were Seneca, Onondoga, Oneida, Cayuga, and Mohawk. For the most part they ceased fighting with one another and become one of the strongest forces in the 17th and 18th century Northeast America. In 1720 the Tuscarora fled European invasion of North Carolina to New York to become the Sixth nation, under the protection of the other Iroquois.
References: "The Ordeal of the Longhouse", by Daniel K. Richter