In the book, the Dwarvish army arrived several days after the Elfish (Elven?) army did. Thorin and Company had fortified the front gate sufficiently strongly to deter a frontal assault, so everyone was dithering. The Men and Elves had declared Erebor under siege. They apparently never gave any thought at all to Dwarvish reinforcements.
IIRC they didn’t realize that the dwarves could communicate via ravens. They thought they had Thorin bottled-up.
Bard, at least, might have known better, having gotten his own vital news from a thrush.
(When I was very young, I was confused, thinking that THRUSH meant “bad guys” because of the Man From U.N.C.L.E. So what did that make Bard’s thrush?)
Just saw this steaming pile of tripe, for free, thankfully. What an overwrought, cliche-filled soap opera. Jackson should be ashamed of this entire effort.
/like