I must say I agree with Lawende and Nerindel when they say that Sauron could have tempted Smaug to do his bidding yet I don't think he could have controled Smaug for too long. Sooner or later, either Smaug would ask too much from Sauron or perhaps Sauron himself would feel he did not need Smaug anymore and try to get rid of him.
Tricksy, these evil creatures are
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