[denethorthefirst]

I get what you mean, but your analogy is not quite right. You cannot compare the Rohirrim with Sauron. Cirion made a pact with good Men, he did not betray his Faith, the founding mission of Gondor and the vision of Elendil. Elendil would approve, at least thats what I think. An alliance with Mordor however would contradict everything the Faithful Numenoreans, the Founders of Gondor, stood for, it would be the ultimate betrayal. So one could argue that, theoretically, Cirion, as Steward of the King, as a caretaker of the Realm and of royal property, did not have the right to give away territory. So he maybe acted against the letter of his "contract" as a steward, but he did not act against the spirit of his "contract", because the pact between Rohan and Gondor did align with the ethos and the culture of the Faithful. You argued in your post that Cirion may have been right in a legal sense but wrong in a moral sense. But one could also argue that the opposite was the case: that Cirions action was, strictly speaking, maybe legally wrong, but in a spiritual-moral sense right.

It is a matter of perspective. We also have to remember that the good relations between Gondor and the Northmen predated Cirion and existed at least since the Reign of King Romendacil II, who further deepened the relationship between Gondor and the Norhmen and even sent his son Valacar to the North, to learn the language and culture of the Northmen. Valacar even married a northern woman (Vidumavi), and their son King Eldacar won the civil war against the usurper Castamir with the help of said Northmen. Thats over a thousand years prior to the Ride of the Rohirrim! And all the Kings after Eldacar seem to have supported that relationship, for we know that the Northmen helped Gondor multiple times in the Wainrider-Wars (and maybe Gondor would have perished then and there without that help). If we take that long and positive relationship between Gondor and the Northmen into account, a relationship that was started and promoted by the Kings (long before Hurin of Emyn Arnen even became the first Steward from the House of Hurin!), then it seems that Cirions action was more or less the logical endpoint of a (thousand year) long foreign policy direction of Gondor.

And what were the alternatives? What should Cirion have done differently? Just give the Rohirrim some gold and then send them their way? Large parts of Calenardhon were still infested with enemies, it would have taken Gondor several years to completely clear the territory, and after all that labour Gondor would not even have been in a position to effectively maintain Calenardhon. If Cirion had acted that way, then Gondor would have been (on paper) larger in territory, but it would have been weaker overall. And it is also quite possible that the good relationship between Gondor and the Northmen would have suffered if Cirion had demanded that they return home. I guess that Cirion rightly anticipated that it would only be a matter of time until the Easterlings renewed their attack to populate the now almost completely empty Calenardhon. And what will Gondor do then? Again call for the Northmen? How willing would they be then to come again to the help of Gondor? Who would be the better neighbor? Better invite the Rohirrim now, than to risk a perhaps unavoidable takeover by the Easterlings in a few decades. In my opinion it was the more prudent decision to accept the Rohirrim as neighbors now, than to leave the territory empty, a defenseless victim for a future takeover.

So, even if Cirions decision might have been "wrong" in a legal or moral sense, he could not have taken another decision from Gondors perspective. Cirions decision safeguarded the future existence of not only Gondor but of all the western lands behind the Anduin. Cirion is a mortal, he lived in the here and now and he was responsible for millions of humans. He simply did not have the luxury do expel the Rohirrim only to satisfy some legalist interpretation of his responsibilities without any real-world benefits. How would that have helped the People of Gondor?