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The Dead fulfilled their Oath. Their Oath was to fight against Sauron, and they fulfilled it by destroying the Corsairs. They could not be made to fight a war; it would go beyond their Oath and wasn't fair to them. That's why Aragorn allowed them to go.
Remember what they said: "To fulfil our Oath and have peace." They wanted to fight and get it over with, so they could finally rest in peace. And Aragorn let them, though certainly it must've seemed tempting to try and make them come with him to Pelennor. But I don't think they would have obeyed him if he had. They had fulfilled their Oath already, and he had no real control over them.
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"If you're referring to the incident with the dragon, I was barely involved. All I did was give your uncle a little nudge out of the door."
THE HOBBIT - IT'S COMING
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