Erkenbrand of the Westfold was given command of the West-mark when news of the fall of Theódred reached the Hornburg the next day.

However, command in the field was given to Grimbold until Erkenbrand could come to the fray. It was expected that Saruman's troops would try to over-run the Westfold and march against the Hornburg.

Grimbold manned the Fords of Isen with footmen from the Westfold. Elfhelm took the position at the eastside, northward of the Fords of Isen, where he believed the attack from Isengard would come.

Saruman's strength was great, thus he began his attack before noon at the western side against the fords with only a small host that even so greatly outnumbering the defenders. The attackers succeeded in forcing Grimbold to withdraw across the fords with great losses. For a time Grimbold held the eastbank of the fords, hoping and waiting for Elfhelm to come in his aid. But there was no sight of Elfhelm. More than half of the remaining army of Saruman was sent down the east-side of the Isen. Without any warning, several troops of Wolf-riders silently and unseen had passed between the host of Elfhelm and Grimbold. The Wolf-riders and their followers were trying to surround Elfhelm and his riders, and the latter were obliged to retreat eastward despite knowing that Grimbold was in need of help.

Grimbold still held his position, but soon he saw torches coming from the north and from Isengard. The vanguard of the remaining forces of Saruman was coming to towards him. They came at great speed and swept over the fords. Grimbold could not hold the Eastbank and withdrew from it, forming a great shield wall about his camp. Soon he was surrounded, though still the enemy was not able to break through the shield wall. Grimbold knew it would not hold forever and still he could not see anything of Elfhelm. He decided to try to break out, knowing that here he would not be of any help to Erkenbrand in the Hornburg.

He mounted all the riders for whom he had horses and set them under the command of Dúnhere. They opened the shield wall at the eastside, and the riders passed through, divided into two units, and attacked the besiegers to the north and south. With the confusion created by this sudden attack, the remaining Rohirrim retreated on foot in the dark with what speed they could. Saruman's commander realised that the shield wall had been broken, and he now had gained control of the fords. The commander had thus obtained his first objective, to secure a safe supply-line into Rohan, and he then made for his primary objective - the Hornburg.