The Hobbit: Novel Summary: Chapter 17

Summary
Bard again seeks to parley with Thorin, showing the dwarf the Arkenstone. Thorin guesses that Bilbo is responsible and shakes him in anger. At that moment, Gandalf shows himself to Thorin. The dwarf, feeling much betrayed buys back the Arkenstone by promising Bard the one-fourteenth share of the reward originally promised to Bilbo. With that, Thorin sends Bilbo away.
The host of dwarves for whom Thorin had earlier called arrives with enough provisions to withstand a long siege. As battle between the dwarves, men, and elves is about to be joined, Gandalf announces, "Dread has come upon you all!" for the goblins and their wargs are approaching. "So began a battle that none had expected; and it was called the Battle of Five Armies, and it was very terrible." The battle rages long and terribly; Bilbo watches, wearing the Ring. Just as the goblins seem to be on the verge of capturing the Mountain, Bilbo sees the Eagles arriving. A stone falls on his head, and he loses consciousness.
Analysis
As the Battle of Five Armies begins, Tolkien's prose takes on an almost biblical tone, anticipating the tone of the latter books of The Lord of the Rings and underscoring the more serious turn that the present volume has taken. Bilbo's horrified reaction to the battle also finds a counterpart in Sam's reaction to his first encounter with men's violence in The Lord of the Rings, and it no doubt relies on Tolkien's own hatred of and horror at war, forged on the French battlefields of World War I.
The eagles' arrival in the darkest part of the battle illustrates Tolkien's theory of the eucatastrophe: a sudden and unexpected reversal, not from good to bad, as the word "catastrophe" means, but from bad to good (indicated by the Greek prefix "eu"-as in euphoria, euphonious, et al.-attached to "catastrophe"). Tolkien held, "The eucatastrophic tale is the true form of fairy-tale [as a genre], and its highest function" (A Tolkien Reader, p. 85). The "sudden joyous 'turn': this joy, which is one of the things which fairy-stories can produce supremely well, is not essentially 'escapist' or 'fugitive.' In its fairy-tale-or otherworld-setting, it is a sudden and miraculous grace: never to be counted on to recur. It does not deny the existence of dyscatastrophe, of sorrow and failure: the possibility of these is necessary to the joy of deliverance; it denies (in the face of much evidence, if you will) universal final defeat and in so far is evangelium [Gk. "good news," or "gospel"], giving a fleeting glimpse of Joy, Joy beyond the walls of the world, poignant as grief" (A Tolkien Reader, p. 86). Readers need not share Tolkien's Christian convictions to appreciate the contribution he makes to the study of the fairy tale. He identifies the eucatastrophe as that element which, in many ways, makes fantastic literature distinctive, and Bilbo's experience of the eagles-his sudden, unexpected, undeserved experience of hope in the midst of hopelessness ("he had seen a sight that made his heart leap.")-demonstrates that Tolkien, the master craftsman, can construct the same experience for his audience.