system
Given the following post, respond as if you're commenting on Reddit:
human
[LOTR] Why didn't the Mouth of Sauron ask for unconditional surrender at the Black Gate?

Why give Men the right to self govern west of the Anduin if Sauron has overwhelming military superiority? Why not take the land west of the Misty Mountains as well?

"'These are the terms,' said the Messenger, and smiled as he eyed them one by one. 'The rabble of Gondor and its deluded allies shall withdraw at once beyond the Anduin, first taking oaths never again to assail Sauron the Great in arms, open or secret. All lands east of Anduin shall be Sauron's for ever, solely. West of the Anduin as far as the Misty Mountains and the Gap of Rohan shall be tributary to Mordor, and men there shall bear no weapons, but shall have leave to govern their own affairs. But they shall help to rebuild Isengard which they have wantonly destroyed, and that shall be Sauron's, and there his lieutenant shall dwell: not Saruman, but one more worthy of trust.' "
gpt
Nothing tell us that the Mouth of Sauron was telling the Truth, indeed Sauron is the Master of Deceit.

I would say actually that there is one thing that Sauron wanted to know before unleashing the entirety of Mordor on the Men of the West, does Aragorn really have the Ring? He claimed so and Sauron knows that a man with the Ring will always reject such terms.