The Desolation of Smaug’s ending occurs after the titular dragon escapes from the Lonely Mountain and flies towards Laketown where he plans to wreck havoc. The camera then cuts back to Bilbo who asks himself what they’ve gotten themselves into. Cut to black.
Jackson revealed in a recent interview with
EW that before they split the movie into three films, Part 1 and Part 2 were going to be split right after Bilbo and the Dwarves see Bard the Bowman’s silhouette in the distance. The second movie would’ve picked up with their journey into Laketown.