Why did Gandalf leave the expedition and leave Thorin and company to go into Mirkwood by themselves in The Hobbit?

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Kristen Lentz eNotes educator| Certified Educator

Gandalf leaves Thorin and company at the edge of Mirkwood.  All the dwarves and Bilbo are very upset and concerned about his imminent departure.  Gandalf, however, will not be persuaded, telling his friends:

"It is no use arguing.  I have, as I told you, some pressing business away south; and I am already late through bothering with you people" (125). 

Although Gandalf does not openly disclose his plans to Bilbo and the dwarves at the time, Tolkien later reveals the purpose of his business in Chapter 19, "The Last Stage."  When Bilbo and Gandalf at last return to Rivendell on their journey home, Bilbo hears Gandalf tell Elrond about his secretive mission.  Gandalf had been to the White Council, which is a powerful council of wizards and elves, including Saruman and Galadriel.  They had met to discuss the growing threat of the Necromancer (Sauron) in Dol Guldur, south of Mirkwood, and the Council decides to use their collective powers to drive him away.