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This question already has an answer here:

I saw the Fellowship of the Ring. Nine rings were forged for the kings of men.

I saw the entire series, the nine are on the side of Sauron.

I am watching the Hobbit : Desolation of Smaug. Gandalf mentions,

The nine only answer to one master

But in Fellowship of the Ring, it is deemed necessary for the one ring to have control over all others (One Ring to rule them all, One Ring to find them, One Ring to bring them all and in the darkness bind them).

My question is why do the wraiths of the Kings do Sauron's bidding? He clearly does not have the One Ring to maintain control. Should they not be outside of the influence of the ring and Sauron?

merged by Rand al'Thor Mar 24 '16 at 1:30

This question was merged with How did Sauron remain in control of the Nazgûl? because it is an exact duplicate of that question.