Thranduil var försiktig
Med nyheterna om Smaugs dödspridning i hela norra länderna, skulle Goblins och Wargs definitivt få vind av det och försöka plundra skatten också (med rätta sagt, eftersom detta hände). Med möjligheten till en strid före Erebor sätter Thranduil en armé med honom för att "hävda sin del av skatten".
Detta anges alla i 2 avsnitt i Hobbit . Ursäkta längden på citatet, jag har fetstil de relevanta punkterna för enkel hänvisning.
Meanwhile Bard took the lead, and ordered things as he wished, though always in the Master's name, and he had a hard task to govern the people and direct the preparations for their protection and housing. Probably most of them would have perished in the winter that now hurried after autumn, if help had not been to hand. But help came swiftly; for Bard at once had speedy messengers sent up the river to the Forest to ask the aid of the King of the Elves of the Wood, and these messengers had found a host already on the move, although it was then only the third day after the fall of Smaug. The Elvenking had received news from his own messengers and from the birds that loved his folk, and already knew much of what had happened. Very great indeed was the commotion among all things with wings that dwelt on the borders of the Desolation of the Dragon. The air was filled with circling flocks, and their swift-flying messengers flew here and there across the sky. Above the borders of the Forest there was whistling, crying and piping. Far over Mirkwood tidings spread: "Smaug is dead!" Leaves rustled and startled ears were lifted. Even before the Elvenking rode forth the news had passed west right to the pinewoods of the Misty Mountains; Beorn had heard it in his wooden house, and the goblins were at council in their caves.
"That will be the last we shall hear of Thorin Oakenshield, I fear," said the king. "He would have done better to have remained my guest. It is an ill wind, all the same," he added, "that blows no one any good." For he too had not forgotten the legend of the wealth of Thror. So it was that Bard's messengers found him now marching with many spearmen and bowmen; and crows were gathered thick, above him, for they thought that war was awakening again, such as had not been in those parts for a long age.
Thranduil säger: " Det är en sjuvind " och " som slår ingen bra på ." Han gissade redan att det skulle vara en strid mot Thrors skatt, så han tar med sig en armé med just det.
Att se att (mest av) filmen Slaget vid de fem arméerna är anpassad från boken, skulle vara lika bra som ett kanoniskt svar som du kan få.