As usually, the answer can be found in the books. In this case, we have the - as I will show - clearly wrong assumption that Aragorn could have taken the army of the Dead further than the Pelennor. However, LotR V,2 disproves this:
“Then Aragorn said: ‘The hour is come at last. Now I go to Pelargir upon Anduin, and ye shall come after me. And when all this land is clean of the servants of Sauron, I will hold the oath fulfilled, and ye shall have peace and depart for ever.”
If Aragorn made a mistake at all, it would have been to set the condition for the fulfillment of the oath like he did, and not extend it. Given that we should assume Aragorn’s knowledge to be greater than ours, we can conclude that the original oath the Dead had broken did in turn NOT extend beyond ‘this land’.
Added to this would be the fact that the Dead Men were hardly an apt army, let alone retinue for a King unless in most dire need: “But defenders and foes alike gave up the battle and fled when we came, crying out that the King of the Dead was upon them. Only Angbor, Lord of Lamedon, had the heart to abide us;” (LotR V,9).
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