In The Hobbit, what was the original purpose of Sauron's army?

Question

In the third Hobbit movie and in the book, the massive army of orcs and goblins from Dol Goldur and Gundabad march on The Lonely Mountain, resulting in the Battle of the Five Armies.

My question is what was the original goal of this force? It is shown several times that Smaug and Sauron were in league together, and that Smaug was presumably waiting for Sauron to gather more strength in order to take over Middle Earth. Sauron wanted the Lonely Mountain both for its strategic position and strength as a fortress, and I assume Smaug was willing to let Sauron use the mountain for that purpose, even if probably wasn't willing to let Sauron have any of his hoard.

So, when Smaug died, Sauron must have had a "oh shit" moment and commanded Azog to go take the Mountain, fast. But if he didn't need that army to take the Lonely Mountain before, what was his target? Azog had a standing army ready to go, so what was it for? The Mirkwood Elves? The Ironhill dwarves? Or just to torch the landscape and turn it into Mordor 2.0?

Answer 1

Lord of the Rings, Return of the King, Appendix A, Durin's Folk.

Among many cares he was troubled in mind by the perilous state of the North; because he knew then already that Sauron was plotting war, and intended, as soon as he felt strong enough, to attack Rivendell. But to resist any attempt from the East to regain the lands of Angmar and the northern passes in the mountains there were now only the Dwarves of the Iron Hills. And beyond them lay the desolation of the Dragon. The Dragon Sauron might use with terrible effect. How then could the end of Smaug be achieved?

Sauron's original purpose was therefore an attack on Rivendell.

This situation doesn't actually exist in the book, The Hobbit, of course. There, the goblin army is quite independent of Sauron (who, as the Necromancer, had just been defeated and driven out by Gandalf and the White Council, of course) and is given a different motive:

Ever since the fall of the Great Goblin of the Misty Mountains the hatred of their race for the dwarves had been rekindled to fury. Messengers had passed to and fro between all their cities, colonies and strongholds; for they resolved now to win the dominion of the North. Tidings they had gathered in secret ways; and in all the mountains there was a forging and an arming. Then they marched and gathered by hill and valley, going ever by tunnel or under dark, until around and beneath the great mountain Gundabad of the North, where was their capital, a vast host was assembled ready to sweep down in time of storm unawares upon the South.

In the book this army was led by Bolg, not Azog; Azog had been slain in Moria by Dain Ironfoot.

Answer 2

In the book (both The Hobbit and LotR)

1. There's no hard proof evidence of alliance between Sauron and Smaug in the timeline of The Hobbit. Sauron was reading himself to "flee" to Mordor, wanting the White Council to believe he's weak and nothing more than a shadow. He certainly would have been glad to gain Smaug to his side, but it's rather difficult to bargain while appearing weak. If Smaug was still alive when he declared himself in Mordor, things may have been quite different on a lot of levels. But as this time, Smaug creating fear in Erebor's region was quite enough for the time being. Tricking the White Council and preparing his return to Mordor was his main objective.

2. It is rather uncertain if the goblins (aka orcs) were under direct order from Sauron. He was probably pulling the strings with his Nazgûhls, but most of them were busy preparing for their master's return in Mordor.
   

   (Gandalf) "Some here will remember that many years ago I myself dared to pass the doors of the Necromancer in Dol Guldur[...]: he was none other than Sauron, our Enemy of old, at length taking shape and power again. [...]
   As his shadow grew, Saruman yielded, and the Council put forth its strength and drove the evil out of Mirkwood. [...]
   "But we were too late, as Elrond foresaw. Sauron also had watched us, and had long prepared against our stroke, governing Mordor from afar through Minas Morgul, where his Nine servants dwelt, until all was ready. Then he gave way before us, but only feigned to flee, and soon after came to the Dark Tower and openly declared himself."
   _{The Lord of the Rings, THE COUNCIL OF ELROND}

Having both of these points in mind, the simpliest reason is: wreak havoc and make prisonners

In the film

Well... it's very uncertain, but an alliance between Smaug and Sauron was not yet made, if at all in bargain.
If nothing else, Sauron's army wanted to the same thing as in every other trilogy, Pinky: try to take over the world.