Iarhen seems to be postulating that Sauron wanted control over Middle Earth in order to bring about the return of Morgoth. Not a bad theory; it has some merit. But my own feeling is that desire for utter and absolute power is what motivated him to wage war against the "free peoples".
He may not have been governed by this motivation from the very outset. In the Ainulindale, we can surmise that Sauron was one of those Ainur who harkened to Melkor's theme and joined in the dissent. This rebelliousness against Iluvatar was present from the start. In the First Age, Sauron began to get a taste for power in his high ranking position in the forces of Morgoth. He seemed to have a certain amount of autonomy in leading the pursuit of Barahir and company, and commanding Tol-in-Gaurhoth. For this reason, and more importantly because Morgoth was far more powerful than him, I believe he was settled in his role. He was a happy employee, if you will. It was during the Second Age, with no Morgoth around, that he really began to come into his own, and seek dominance, first over the men of Middle Earth, then over the Numenoreans. By this time, he would have felt a lot of hatred towards the Eldar and Edain, those who had fought gallantly against him in the First Age. Also reinforced by the fact that they were favoured by the Valar, whereas he was now an outcast because of his actions. Rehabilitation and acceptance was never an option for him.
So, what I'm saying is that, with these things taken into consideration, his hatred, and his growing lust for power (if I may paraphrase one of my favourite movies, all those who gain power are afraid to lose it) are enough to explain why he wanted to conquer Middle Earth. I don't believe that he would have been seeking the return of his former master. I feel he would have been reluctant to once again assume a subservient role, as his ambition had grown in the interim. Or, as
Willie says, perhaps his daddy decided to hug the mailman?!
