I've decided on preemptive self-defense(a great word from last year...
) and post this before Rumil's wonderful in-depth look on...
The First Battle of the Fords of Isen.
Because of Saruman's position, a double envelopment of the Fords of the Fords could be achieved with ease. The problem of his assault was improper disposition of his massive resources. For once the Fords are captured, it would have been impossible for the Navy-less Rohirrim to send reinforcements for the western garrison. Thus, the western army would be safe from any flank or rear attack.
The eastern army is another story. Since it is inside the boundary of Rohan, a force could be sent against it from anywhere, except along the line of the river. If the Rohirrim mustered enough force, they could encircle the troops sent on the east shore(by the forces from the Fords and the newcomers), then destroy the western division with an overwhelming concentration of men.
Had Saruman properly disposed of his forces(with a larger eastern army), he could have repelled Elfhelm's charge, or at least held it off long enough to destroy Théodred's beleaguered men at the Fords.
Later days!
->Elenrod