system
Given the following post, respond as if you're commenting on Reddit:
human
[LOTR] Why didn't Aragorn stipulate he wanted the undead army to help him defeate Sauron and his armies then they would be released of their oath?

He got them to attack the ships for one. He got them to help him at the Battle of the Pelennor Fields. Why not get them to help one step closer at the gates or Mordor? It's not like they killed all those people on the ships and were like. "We're done, can't help you any further."

And most of all it's not like they had anything better to do anyway. They are undead. 

Why stop? Why not go one last step to help destroy in my opinion, the most important force of ork reserves (10,000) if Frodo and his quest failed? 

They would have cut through them like cheese. 

Why didn't he simply say, "Help see us all through this and you are free."
gpt
Basically, he didn't want to push his luck with them. The possibility existed that at a certain point they would feel they had fulfilled their obligation and abandon him or turn on him, or that Sauron might manage to seize control of him, meant that he dared not take them further than to battle the Corsairs of Umbar (book) or to help defend the Pelennor fields (film)