In an interview with Empire to promote BBC's Sherlock, Benedict Cumberbatch commented on his roles of Smaug and the Necromancer, revealing a tiny plot change.

"I’m playing Smaug through motion-capture and voicing the Necromancer, which is a character in the Five Legions War or something which I’m meant to understand. He’s not actually in the original Hobbit. It’s something [Peter Jackson]’s taken from Lord Of The Rings that he wants to put in there."

In the novel, the Battle of Five Armies (his Legions reference) is essentially a fight over Smaug's treasure after his defeat. The Necromancer (aka Sauron) was defeated in a battle at Mirkwood that occurred "off screen" halfway through the novel. In the book, Gandalf left Bilbo and co to their own devices why taking care of an unspecified adventure that was revealed in later books to be a fight with
Sauron.

The comment above suggests either a few things. One is Peter Jackson merged the two battles, giving a greater purpose to the Five Armies battle beyond simple greed. I think the more likely possibility is both battles are depicted in the movies (Mirkwood in Unexpected Adventure, Five Armies in There and Back Again), with Mirkwood being the first fight with Sauron and the Five Armies being the second while retaining its significance as would provide a nice transition to Sauron rising to power in Mordor for The Lord of the Rings. This fits with what little we know of filming as the departure of Gandalf is kept in the movie (per an AICN set report) and while providing a significant battle for the first movie, something the book didn't have. If true, Tolkien purists will pitch a fit but I doubt movie goers will complain.

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