What factors contributed to the decline of the powerful self-reliant Iroquois Indians?

The Iroquois established a unique federation of nations on the American continent that saw great peace and prosperity.  The arrival of English colonists caused the decline of this once prosperous people for a number of reasons. The English brought with them a myriad of diseases for which Native Americans did not have an immunity.  Smallpox, measles, and influenza were particularly devastating to the population of the Iroquois during the Seventeenth Century. In addition to the diseases brought by the English, political and economic uncertainty and turmoil would follow.  The competition for furs between European countries plunged the Iroquois into a number of conflicts that cost the tribes lives on the battlefield.  Death from warfare and disease coupled with unfair treaties with the new American government ultimately led to the demise of the Iroquois.  
https://www.everyculture.com/multi/Ha-La/Iroquois-Confederacy.html