Question

what may have strengthen Iroquois leaders resolve not to ally with britain

Answer 1

Defeats of the British at Fort Niagara and Lake George

(Hope this helps)

Answer 2

The Iroquois decided not to ally themselves, probably because of the defeat that Great Britain  suffered in Fort Niagara and Lake George.

The Iroquois were a Native American group that lived in the Great Lakes region where today is the state of New York in the United States. Five nations - caugas, mohawks, oneidas, onondagas and sínecas - originally constituted the Iroquesa Confederation.