There are no terrible alien creatures in the middle of the earth, and in the days of the Hobbit, Dragon Smak was soon one of the most powerful of all.
Yet few can question Sauron's power, with or without one ring. So if Smaug were still alive during the Lord of the Rings, would the dragon in Sauron's army be forced, as many other evil beasts before him had done?
While Smaug slept in Erebor, he had no idea that Sauron was returning to Dol Guldur Fortress.
Over time, those with bad intentions seized the power of Sauron and came under his rule, with pleasure and reluctance.
But was it easy to defeat with Sauron's power?
While the Dark Lord is very powerful, capturing a dragon can be a daunting challenge. Smaug could easily cut off Mordor's armies and delay the invasion of the Mediterranean for years.
So it was not in Sauron's interest to take the dragon, but that did not mean that he considered it a lost cause because he fully intended to form an alliance.
In the book Unfinished Tales of Númenor and Middle-earth, many pieces of Tolkien's writings are combined into short stories.