If the majority of movie villains weren’t bound to take each other out before being able to conquer their enemies, as well as the insane amount of plot armor heroes have, then the results would no doubt be catastrophic. The type of damage that some enemies could do would be hard to come back from, as various heroes know, and it’s very easy to think that if certain villains were to work together that they could lay waste to an entire countryside, perhaps all of Middle Earth had they been given the chance. Trying to imagine the overall damage that Smaug and Sauron could have done had they been able to ally with each other is tough to think about. Obviously, Sauron had been weakened greatly by the time Smaug met his end, but the idea of what could have happened had Smaug not been killed by Bard, and if Sauron could have enticed the dragon to join his cause, is simply chaotic. Thinking of how the dragon could lay waste to one countryside after another without fear of any more lance-like arrows, and how Sauron could command armies of thousands, perhaps even millions, makes it clear that the heroes would need a miracle to win.
It’s very likely that had Tolkien ever decided to do this that there would have been a serious need for heroes that might make Gandalf look basic since even with his magical might, Gandalf would have been far too pressed to change the tide of battle in this case. Humans and elves would have died by the hundreds in every engagement, and with an aerial assault, it’s not hard to think that Helm’s Deep would have lasted long. The overwhelming force of Smaug and Sauron is too great to think about since the combined power that each individual wields is enough to overwhelm a sizeable force.
The one drawback of this pairing is one of the most obvious ones since Smaug is insanely greedy and probably wouldn’t be willing to join Sauron’s forces for the sheer fun of it. His greed and arrogance are made quite clear in the Hobbit movies, and it’s not tough at all to think that if Smaug did decide to ally with Sauron that his price would be kind of steep. Would Sauron pay it? Probably, especially considering that the lord of Mordor would no doubt see the inherent value. A dragon the size of Smaug would do a great deal of collateral damage just passing by, since all but the sharpest spears and swords, as well as arrows, would bounce off of his reptilian hide, and a sweep of his claws and his tail would clear the battlefield for several meters in every direction. Imagine Smaug landing and executing a whirlwind attack, which is something that might be recognized by those that have played D&D long enough. Breathing fire, whipping his tail, biting at enemies, using his claws, and using his wings in a type of hurricane effect are all ways that Smaug would be able to terrorize those on a battlefield. But his mere presence has been enough to scare the living hell out of people as many fans already know.
Adding the presence of Sauron to that type of power would have been an insurmountable force to most opponents, though it’s fair to think that those under the mountain, the ghosts that Aragorn ended up freeing at one point, might have evened the odds. One thing is certain, with the elves fading and Gandalf being the only powerful wizard in the fight, it’s fair to think that Middle Earth would have been trembling in a big way, at least until someone could come up with a way to counter Smaug. It would have made for an interesting fight, to say the least, since Smaug would have probably been careless enough to roast or trample portions of Sauron’s army, which would have lent a comical moment or two that fans might have liked. The truth about Smaug is that he’s not a stupid character, but he’s not an idiot. The dragon knows how to fight, and he knew how to devastate a given area. But that arrogance would probably still be his downfall had he survived to the events of the trilogy that came next.
The dragon is simply too confident in his abilities, but for good reason. What would have also stood between Smaug and Sauron no doubt is the fact that greed is what Smaug is all about, while Sauron is about domination and power. Whether he would have cared if Smaug burned all of Middle Earth to the ground is hard to say. But it’s not very likely that Smaug would have joined Sauron without any questions, or payment before getting the job done. But the pairing alone would be trouble if it did happen, so it’s all the better that it never did.