The Two Towers
How we cite our quotes:
"I serve only the Lord of the Mark, Théoden King son of Thengel," answered Éomer. "We do not serve the Power of the Black Land far away, but neither are we yet at open war with him; and if you are fleeing from him, then you had best leave this land. There is trouble now on all our borders, and we are threatened; but we desire only to be free, and to live as we have lived, keeping our own, and serving no foreign lord, good or evil. We welcomed guests kindly in the better days, but in these times the unbidden stranger finds us swift and hard." (3.2.124)
Gondor is obviously committed to the anti-Sauron fight, but Rohan's position is, in a way, more interesting. According to Éomer, the people of the Riddermark just want to be free, and to mind their own business, thank you very much. They aren't openly committed to either Good or Evil, the way the Elves and the descendants of the Men of Westernesse are. Still, Rohan is going to get pulled into this conflict whether they like it or not.
But Isengard cannot fight Mordor, unless Saruman first obtains the Ring. That he will never do now. He does not yet know his peril. There is much that he does not know. He was so eager to lay his hands on his prey that he could not wait at home, and he came forth to meet and spy on his messengers. But he came too late, for once, and the battle was over and beyond his help before he reached these parts. He did not remain here long. I look into his mind and I see his doubt. He has no woodcraft. He believes that the horsemen slew and burned all upon the field of battle; but he does not know whether the Orcs were bringing any prisoners or not. And he does not know of the quarrel between his servants and the Orcs of Mordor; nor does he know of the Winged Messenger. (3.5.98)
As Gandalf is explaining the Saruman situation to Aragorn, Legolas, and Gimli, we see the damage that pride does to a person. Saruman is so certain of his own awesomeness that he can't see where he's weak. In isolating himself from all possible allies (except his buddy Sauron, whom he is also trying to betray), he has cut himself off from networks of information that would really help him out. Whatever you may say about Sauron, at least he's smart enough to have Winged Messengers. Saruman doesn't have any lieutenants who he can really trust—not even wretched, craven Gríma Wormtongue.
"A man may love you and yet not love Wormtongue or his counsels," said Aragorn.
"That may be. I will do as you ask. Call Háma to me. Since he proved untrusty as a doorward, let him become an errand-runner. The guilty shall bring the guilty to judgement," said Théoden, and his voice was grim, yet he looked at Gandalf and smiled and as he did so many lines of care were smoothed away and did not return. (3.6.81-2)
Gríma Wormtongue is so successful at wreaking havoc in Rohan because he managed to isolate Théoden from all the folks who might disagree with Wormtongue's own sneaky advice. For example, he gets Théoden to toss his extremely loyal nephew, Éomer, into jail, just so he's the only one whispering in the King's ear. But the reversal of Wormtongue's evils is also remarkably quick: as soon as Gandalf restores Théoden's allies to him, and as soon as Gandalf brings him new allies (Aragorn, Legolas, and Gimli), he perks up visibly—those "many lines of care were smoothed away and did not return." Companionship is like the Fountain of Youth for Théoden.