Battles of the late Third Age

[Numenorean]

Hi Tuor, you asked:

why would a Rohirric rout in far western Rohan necessarily have been decisive, given that the basis of a 10, 000 + Rohan force still presumably existed in central and eastern Rohan, and you would think that a considerable portion could have been mustered in the Edoras/Dunharrow area.

I see your point, but I was just extending the premise that JRRT lays out in U.T, before the account of the battle he says:

Saruman made the mistake, fatal as it proved, of not immediately throwing in more forces and proceeding at once to a massive invasion of Westfold; though the valour of Grimbold and Elfhelm contributed to his delay. If the invasion of Westfold had begun five days earlier, there can be little doubt that the reinforcements from Edoras would never have come near Helm's Deep, but would have been surrounded and overwhelmed in the open plain; if indeed Edoras had not itself been attacked and captured before the arrival of Gandalf.