if Smaug and Sauron did form an alliance what would have been the outcome of the war? Could Sauron control Smaug?
This bit from the "Unfinished Tales" entry on Wikipedia will hopefully help:
"The Quest of Erebor" is written in the first person, from the perspective of Frodo Baggins However, nearly all the text consists of narration by Gandalf, who was telling the story at the request of Frodo in Minas Tirith after the coronation of King Elessar
Gandalf knew that Smaug the Dragon could pose a serious threat if used by Sauron then dwelling in Dol Goldur in Mirkwood. He was thinking about the matter when he met Thorin Oakenshield at Bree"
Sauron's master, Morgoth, created the dragon species and they were his servants, so Sauron had a pretty good chance at either commanding Smaug, or at least teaming with him.