Why did Thranduil bring an army to Erebor?

Question

When the Dwarves fled Erebor following Smaug's invasion, Thranduil was seen arriving with an army before turning away promptly to return to Mirkwood.

Why did he bring an army to Erebor? Did he genuinely intended initially to reinforce the Dwarves in defeating Smaug? More questions come to mind if this possibility is considered: How much time transpired between the first sign of Smaug's coming until the fleeing of the Dwarves? Was there enough time for Thranduil to be informed of Smaug's attack, amass an army and then arrive at Erebor in time to see the Dwarves flee?

If the above is not the case, then what other possibilities are there? Did he originally plan to attack Erebor to take back "something of mine", the same reason motivating him to march an army to what would become the Battle of the Five Armies? Or was he merely passing through en route to home? If so, then where was he bringing an army from originally?

Or are there any other possible explanations?

Answer 1

It depends on whether you're talking about the book, or the movie.

In the book, we see absolutely nothing about the Elves when Smaug first invades Erebor. They only enter the story when the party goes to Mirkwood. In the movie, there are many, many events that the books do not mention, and this is one of them.

In the book, the only army of Elves that is seen is when Thorin escapes the Elvenking and goes to Laketown. Thranduil first heard of Thorin's arrival in Laketown from the raft-men of the Elves, who were sitting at the lower tables at feast. Indeed, when Thorin announced himself to the Master of Laketown, the Elves came forward and told the Master:

"These are prisoners of our king that have escaped, wandering vagabond dwarves that could not give any good account of themselves, sneaking through the woods and molesting our people!"

And later, we see Thranduil's reaction:

In the meanwhile the Wood-elves had gone back up the Forest River with their cargoes, and there was great excitement in the king's palace. I have never heard what happened to the chief of the guards and the butler. Nothing of course was ever said about keys or barrels while the dwarves stayed in Lake-town, and Bilbo was careful never to become invisible. Still, I daresay, more was guessed than was known, though doubtless Mr. Baggins remained a bit of a mystery. In any case the king knew now the dwarves' errand, or thought he did, and he said to himself:

"Very well! We'll see! No treasure will come back through Mirkwood without my having something to say in the matter. But I expect they will all come to a bad end, and serve them right!" He at any rate did not believe in dwarves fighting and killing dragons like Smaug, and he strongly suspected attempted burglary or something like it which shows he was a wise elf and wiser than the men of the town, though not quite right, as we shall see in the end. He sent out his spies about the shores of the lake and as far northward towards the Mountains as they would go, and waited.

Thranduil brought an army to Erebor to see what part of the treasure he could claim.

Answer 2

It is a clumsy attempt to picture the grudge between elves and dwarves.

Let's remember that this aspect is never covered in the LOTR movie: why the mistrust exists is never explained.

There are the usual puns between Legolas and Gimli, but they are not hostile nor ill-intended. Both characters are also good-hearted and not spiteful, as some other of their races could be.
At Elrond's Council, in the movie only, the dwarf grudge is loudly heard when Gimli declares "I will be dead before I see the Ring in the hands of an Elf". It is clear the ring divides everyone here and deepens the mistrust, which is swayed away when Frodo suggests he takes the Ring.
There is the scene of Moria's door, where the long gone better days of trust between elves and dwarves are mentioned, but they are not detailed.
There is the Lorien passage, where the whole company has to walk blind-folded because of the mistrust of the elves for the dwarf. But this mistrust is never really explained and can be seen as "we didn't expect any visitors, least of all a dwarf".

For anyone who has played some fantastic and medieval RP games, this legendary grudge between the two races is rather well acknowledged and different explanations are given from one universe to another.

Since the movie is aimed for anyone, whether they've seen LOTR or not, it is important to set some context. In our modern times, showing why the two races don't trust each other is more important than a well-cut dialogue.
Also, because the scene is visible at the very beginning of the movie, people will more easily remember it later on.

As for the in-movie appearance of the elven king:
Considering that the outskirts of Mirkwood are at some distance of Erebor, that Smaug's blitz attack should not have left much time for the elves to hear the news, let alone prepare and mount an entire army, one can only presume that Thranduil was on a leisure stroll with a small guard contingent.

Answer 3

To help fight the dragon

Yes, this is a a highly-speculative, in-Jacksonverse answer. But here it goes.

When Smaug attacks Dale / Erebor, the dwarves send out a distress call. Thranduil's people would have found out about it sooner or later via messengers.

Why would Thranduil want to help?

The relationship between the wood elves and the dwarves wasn't completely soured at the point. But also, they had a good relationship with the men of Dale (which continued once they moved to Lake Town). So Thranduil would have wanted to go for their sake too.

Thranduil wasn't risking too much. Smaug was the last of the dragons, but there were dragons / drakes before him that we can assume were not as dangerous, and which were most likely killed by the dwarves, men of Dale, and/of the wood elves, because:

1. the dwarves had a protocol for fighting dragons, which they activated when Smaug attacked (of course it didn't work against him)

2. the men of Dale has anti-dragon defenses (the "wind lances").

3. in Dale they had dragon-shaped kites. This shows they knew about dragons. I think this also shows that they didn't take the threat too seriously, because the previous dragons were not as dangerous as Smaug.

If Thranduil had known that the dragon was as powerful (and near-invincible) as Smaug, he would have refused to go.

So why didn't he try to fight the dragon one he arrived?

1. He was too late. The men of Dale had already evacuated, and the surviving dwarves were in the process.

2. Thranduil wasn't going to attack the dragon without help from either the men or the dwarves.

3. Seeing the destruction caused by the dragon, and the high number of dwarven casualties, Thranduil would have realized that this dragon was exceptionally dangerous and probably not beatable by his forces.

4. The dragon was entrenched in the Lonely Mountain at the point. With only one known entrance, any attack would be suicide.

Answer 4

Thranduil was being precautious

With the news of Smaug's death spreading throughout the northern lands, the Goblins and Wargs would definitely get wind of it and try to plunder the treasure as well (rightly said, seeing that this did happen). With the possibility of a battle before Erebor, Thranduil brings an army with him to 'claim his part of the treasure'.

This is all stated in 2 paragraphs in The Hobbit. Apologies for the length of the quote, I have bolded the relevant points for easy reference.

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 states: "It is an ill wind" and "that blows no one any good." He already guessed that there would be quarrel over the treasure of Thror, so he brings an army with him just in case.

Seeing that (most of) the movie The Battle of the Five Armies is adapted from the book, this would be as good as a canonical answer as you can get.

Answer 5

This arrival of the Elves was result of the insult the King of the Dwarves did to the Elf king in the extended version of the film.

Answer 6

A lot of good answers here but I'm going to put in my personal take:

It's a Metaphor

Thranduil wasn't there and neither was his army. The appearance and departure in the flashback is a metaphor for their conspicuous absence during the attack. The flashback is from Thorin's perspective and he's clearly biased and irrational, his own memories or retellings shaded by his grudge against Thranduil and the elves as a whole.

Bilbo's own narration (presumably from his writing) only states that no help came from the elves that day, not that Thranduil arrived to spectate, or made a show of turning his back on the Erebor.