The Hobbit: Novel Summary: Chapter 15

The thrush who told Bard how to bring down Smaug now overhears Bilbo and the dwarves talking about the ancient friendship that existed between Thorins ancestors and ravens. The thrush leaves, then returns with an old raven who relays the news of Smaugs death to the company. The raven also informs them of the forces of elves and men gathering to take the dragons treasure for themselves. The raven advises Thorin to trust Bard instead of the Master. An angry Thorin asks the raven to send word to other dwarves, that they might assist him in defending the treasure. The raven departs, and the dwarves and Bilbo head back to the Lonely Mountain. At length, the elves and men arrive, surprised to find Thorin and the others still living. Bard requests negotiations with Thorin, pointing out that his own ancestors wealth is mingled with the dwarves in Smaugs hoard. Thorin adamantly refuses negotiations, however. Bard later gives Thorin an ultimatum: deliver to him one-twelfth of the treasure, from which Bard himself will assist in the rebuilding of Esgaroth, or be considered the foe of the lake-town. Thorin refuses, and the men and elves declare the Lonely Mountain besieged.
One recurring motif in this chapter is Bilbos failure to understand that, even with the death of Smaug, the adventure and his part in it are not over. As his nephew Frodo will be in The Lord of the Rings, Bilbo has been thrust into an adventure-that is, into a life-far larger than he had bargained for. As do all of the characters of Middle-earth whom Tolkien evaluates positively, however, Bilbo “soldiers on,” doing what must be done, regardless of personal feeling. Bilbo complains, to be sure, but he does not renounce his companionship with the dwarves and begin heading home to the Shire. This decision further alerts readers to his characters growth. It also reinforces Tolkiens unspoken but unmistakable commitment that engagement with the world is preferable to withdrawal from it.
Readers should also note the almost-passing reference that Bard makes of the newly forged friendship between elves and men surrounding the rebuilding of Esgaroth. Bard is offering the same chance at friendship to Thorin. Were Thorin somewhat less consumed by pride and greed than he is, he might be able to accept that offer. The siege of the Lonely Mountain thus functions as a cautionary episode, one of many in The Hobbit warning against the potentially destructive power of pride and greed (see Analyses for earlier chapters).