The purpose of the Iroquois League, as indicated in the quote, was primarily to work together and defend each other in order to keep peace. The text speaks about planting the "tree of the Great Peace," which symbolizes unity and cooperation among the Five Nations. The watchful eagle at the top of the tree represents vigilance against threats, and the council fire signifies the ongoing collaboration and shared governance.
Therefore, the correct answer is: B to work together and defend each other to keep peace.