Why Gandalf Is Allowed To Fight Saruman, But Not Sauron, In The Lord Of The Rings

Gandalf was clearly a powerful wizard in The Lord of the Rings, but his power also inspires confusion considering he never used it to fight true evil like Sauron, despite getting into a conflict with others like Saruman. Fans of The Lord of the Rings are aware of the fact that the lore goes so much