Q: Did Sauron Ever Attempt to Form an Alliance With Smaug?
ANSWER: To date, I have seen no published evidence that J.R.R. Tolkien considered Smaug to be anything other than an independent actor in Middle-earth’s conflicts. However, there is the one passage in “Durin’s Folk”, published in The Lord of the Rings, where Tolkien wrote:
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?
I think the implication is that Sauron could indeed have reached out to Smaug at the appropriate time. But apparently that time had never come.
Of course, Smaug’s attack on Erebor occurred when Sauron was disguised as the Necromancer in southern Mirkwood and Sauron was actively working against the fortunes of the Dwarves, just as he had worked against the fortunes of the Dunedain. So it may be that in the back of his mind Tolkien considered Smaug’s attack to somehow be the result of Sauron’s machinations. Sauron might only have needed to make sure that Smaug heard rumor of Erebor’s wealth.
I think people writing fan fiction or role-playing adventures would probably find favor among readers if they devised some connection between Smaug and Sauron, even if it’s no more than a conversation between the dragon and an emissary from the Necromancer (or the Necromancer himself).