Was Sauron Afraid of Smaug? What Really Connected the Villain with the Dragon

Their union would be difficult to control.
In the epic saga The Lord of the Rings, the forces of good win largely due to unity — elves, humans and dwarves, having forgotten old feuds, unite against a common enemy. But the dark side, as often happens, suffers from disunity.
Sauron, the main antagonist, wages war almost alone. Only at the end does Saruman join him, and even then not for long. But if Sauron had made an alliance with the same Smaug, the fire-breathing dragon could have turned the tide of the war.
It is enough to remember what damage the Nazgul did on their winged creatures. And if a real dragon had intervened, it would have become a real "winning card" for Sauron.
But instead of enlisting the support of this powerful ally for hundreds of years of neighboring in Dol Guldur even before the events of The Lord of the Rings, the Dark Lord preferred to stay away from Smaug.
And there were good reasons for that — Smaug's independent nature made him an unreliable partner. This dragon recognized only his own will. It is unlikely that he would obey Sauron — he would rather burn him at the first attempt to give an order.
Or the Dark Lord would have to use any manipulation and promises of wealth, but he would still have to wait for a stab in the back.