See below a passage from Unfinished Tales (The Battles of the Fords of Isen, Note 2, page 364). (I recommend you re-read the first section of The Battles of the Fords of Isen to refresh your memory.)
The Ents are here left out of account, as they were by all save Gandalf. But unless Gandalf could have brought about the rising of the Ents several days earlier (as from the narrative was plainly not possible), it would not have saved Rohan. The Ents might have destroyed Isengard, and even captured Saruman (if after victory he had not himself followed his army). The Ents and Huorns, with the aid of such Riders of the East-mark as had not yet been engaged, might have destroyed the forces of Saruman in Rohan, but the Mark would have been in ruins, and leaderless. Even if the Red Arrow had found any one with authority to receive it, the call from Gondor would not have been heeded – or at most a few companies of weary men would have reached Minas Tirith, too late except to perish with it.[Author’s note.] – For the Red Arrow see The Return of the King V 3, where it was brought to Théoden by an errand-rider from Gondor as a token of the need of Minas Tirith.
Here it is stated that if the Ents had gone to Rohan after rising, despite what they might have done to help Rohan, the Mark would still have been 'in ruins, and leaderless'. How would the Mark have been 'leaderless'? Was Théoden not still alive at the time of the rising of the Ents? Or had he died beforehand? Or, indeed, is the passage referring to the death of Théodred?
Some enlightenment would be appreciated. Thank you.