The Battles of the Isen were fought between Saruman's forces and those of Rohan. The key to the battles was the Gap of Rohan, where the river was broad and shallow, and divided itself into two arms about a large eyot. Only there was the river passable for large or heavily armed forces.

Saruman had the advantage, for he could send forces down either side of the Isen to attack the Fords of Isen and then retreat back to Isengard. The Rohirrim, on the other hand, had to retreat back over the narrow ford with the enemy at their heels, and possibly also with an army awaiting them at the other side of the ford.

Theódred, Theóden's son, was warned about the mustering of troops before the gates of Isengard by his scouts. However, this mustering seemed only to be at the western side of the Isen and he, therefore, manned the eastern and western sides of the ford with men on foot from the Westfold, leaving three companies of riders on the eastern side. Early in the misty morning he passed over to the western side with his main force of cavalry and archers, intending to overthrow Saruman's army before it was fully prepared.

But Saruman had tricked the scouts. When Theódred had passed over the ford, the army of Saruman was already on the march towards them. Twenty miles north of the ford, Theódred encountered them and scattered their vanguard who suffered great losses. But when he rode to attack the main host, they were already prepared for the attack and defended themselves behind trenches manned with pikemen. In addition, new forces came from Isengard, outflanking Theódred and the leading éored on both sides, nearly surrounding them. Theódred commanded a retreat, but the enemy was not shaken off. They were not far behind and Grimbold, who was defending the Rohan rearguard, often had to turn around to drive the most eager of the pursuers back.

The mists of the morning had by then been driven away, and, as Theódred looked eastward, he could see a host from Isengard hastening towards the ford at the other side of the Isen. When he reached the ford, he set Grimbold in command of the garrison of the western bank, and what he had left of riders and horses, he sent to the other side. He stayed behind with his own company. With this company dismounted, he manned the eyot to cover the retreat of Grimbold, if he should be driven back.

The eastern force of Saruman was a much smaller force than the one at the western side of the river, but it was much more dangerous. In this force there were Dunlendings on horses, the fierce Uruk-hai from Isengard and the Wolfriders. The small host of Rohirrim on the eastern bank was driven southward from the ford along the Isen, with the Uruk-hai in hot pursuit. As soon as he Isengarders had gained possession of the eastern bank, they hastened to the eyot and assailed it from both sides. At the same time, Grimbold was being attacked on the western side. As Grimbold looked to the east, he saw Theódred's men being driven from the shores of the eyot to a knoll upon its centre.

At once Grimbold took a few men near him and ran towards the eyot and Theódred, hurriedly cleaving his way through. But he was too late, and when he and his men reached Theódred's position and came to his side, Theódred had fallen. He was hewn down by a great Orc and no one could save him. Grimbold slew the Orc, then stood over Theódred and defending his body from the Orcs. Grimbold, too, would soon have died, if it were not for Elfhelm.

Elfhelm was leading four companies against Helm's Deep, but it was reported to him that two Wolf-riders had been seen abroad in the field. He rode with full speed to the ford. As he came near, he heard from the fugitives of the eastern bank what was happening. He then ordered his companies to charge on sight. The Isengarders were now to be surprised by the sudden attack. Most of the enemy fled northward towards Isengard, and soon he had the eastern bank. With all haste, he rushed to the eyot, thus coming to aid of Grimbold, who was fighting two great Orcs for the possession of Theódred's body. Elfhelm slew one at once and the other fell before Grimbold.

Elfhelm and Grimbold, on lifting the body, found that Theódred was still alive. Theódred, however, he only lived long enough to speak his last words:

"Let me lie here- to keep the ford until Éomer comes".

The attack on the west side ceased and the enemy faded away into the dark. The Rohirrim held the ford, but their losses were heavy. The King`s son was dead and they were now leaderless.