Complex sentence: Kelsey's peace initiatives failed and he was unable to open trade relations with the Naywatame because they were afraid to attempt a journey across their enemy's, the Assiniboine, territory to come to York Factory.
Split into simpler sentences: Kelsey's peace initiatives failed and he was unable to open trade relations with the Naywatame. They were afraid to make a journey across their enemy the Assiniboine's territory to go to York Factory.