In J. R. R. Tolkien's fictional universe of Middle-earth, the Fords of Isen were fords in the river Isen, guarded by the Rohirrim. As the only crossing of the Isen into Rohan, they were of enormous strategic importance.
During the War of the Ring there were two battles at the Fords of Isen. The first battle took place on February 25, 3019 of the Third Age. The second was on March 2, T.A. 3019. The battles were between Saruman's army and the Rohirrim of Rohan.
Théodred, son of King Théoden of Rohan, was alerted by his scouts to the mustering taking place in Isengard. He manned both sides of the ford and went forward leaving three companies of riders to guard the eastern side. Early in the morning, he crossed over to the western side with a force mainly consisting of archers and cavalry. He planned to take the forces of Isengard by surprise.
However, Saruman had tricked the scouts; his army was already marching out to attack the ford. Twenty miles to the north of the ford, Théodred encountered the vanguard of the Uruk-hai and quickly cut through their lines. He then charged at the main force, who were prepared for the attack and were stationed behind trenches planted with pikes. Reinforcements came out of Isengard and outflanked the cavalry, nearly surrounding them. Hastily Théodred ordered a retreat, but the Uruk-hai could not be shaken off that easily. Grimbold, who was in command of the Rohirrim rearguard, had to cut down their pursuers many times.
Saruman's eastern force was much smaller but more dangerous. It contained mounted Dunlendings, Warg riders, Uruk-hai and half-orcs (or orc-men). The Rohirrim guard on the eastern side of the ford had been driven into retreat, and their attackers recrossed the ford to attack Théodred on both sides. Théodred and his men had dismounted on an islet to cover Grimbold's retreat. At the same time Grimbold's force was being attacked from the western side, and as Grimbold looked east, he saw Théodred's force being driven from the islet to a hill. Grimbold and a few men raced toward Théodred's position. By the time Grimbold reached him, Théodred had fallen, cut down by a great orc-man. Grimbold slew the orc-man and then found himself defending Théodred's body from orc-men. He himself would have been killed had it not been for Elfhelm.
Elfhelm had been leading four companies towards Helm's Deep, when it was reported to him that two Warg Riders had been spotted. He rode at full speed to the ford and, seeing the situation, ordered his men to charge. Before long his men held the western side of the ford. They then charged on the islet. From this surprise attack most of the Uruk-hai retreated toward Isengard. When they reached the islet they found Grimbold defending Théodred's body against two huge orc-men. Elfhelm rushed to his aid and felled one orc-man while Grimbold slew the other.
When they lifted Théodred's body, they found he was still alive. He lived long enough to say, "Let me lie here - to keep the ford until Éomer comes". The enemy attack ended by nightfall. The Rohirrim had held the ford but suffered heavy losses and were now leaderless.
The command of the ford was given to Erkenbrand of the Westfold. However, until he arrived from Helm's Deep, Grimbold held the position. Elfhelm did not want to hold the Fords, as he argued that it provided little defence, but Grimbold was not willing to wholly abandon it, partially due to the tradition of Westfold. The two commanders later reached a compromise.
Grimbold decided to place foot soldiers at the ford and put Elfhelm's men on the east side where he expected the attack to come from.
Saruman sent a small force, but which was still enough to outnumber the defenders. The attackers forced Grimbold to retreat across the ford with heavy losses. Grimbold held the eastern side of the ford and waited for Elfhelm to come to his aid. More than half of Saruman's force was attacking Grimbold's position.
Some warg riders and their followers pushed through the gap between the two Rohirrim forces and tried to surround Elfhelm. Although he knew Grimbold was in danger, Elfhelm retreated eastwards. Grimbold was still holding his position when he saw torches coming from the north and from Isengard - the vanguard of Saruman's reinforcements advancing towards him. Before he knew it, they had crossed the ford. He could not hold the ford so he retreated to his camp and made a shield wall around it.
Even though the Rohirrim were surrounded, the forces of Isengard could not break through. Grimbold knew he could not hold out forever. With no sign of Elfhelm and no help from Erkenbrand, he decided to try to break out. He mounted all the riders for whom he had horses and made a gap on the east side of the wall through which the riders passed. They formed into two groups and attacked the north and south sides simultaneously. In the ensuing confusion, the remaining Rohirrim retreated on foot as quickly as they could in the dark.
Saruman's commander had finished his first task of creating a supply route into Rohan and he now moved on to his more important task—the Hornburg.
However, Saruman had overestimated his success in the battle, as the Rohirrim had suffered significant losses but not as many as had been rumoured. The surviving Rohirrim from the Fords were later rallied by Gandalf, with Grimbold's men joining up with Erkenbrand to organize the counter-attack at the Hornburg. Elfhelm's riders were sent to Meduseld to guard against any possible raid by Saruman's wolf-riders, though ending up no sortie was attempted.
In the live-action film The Lord of the Rings: The Two Towers, the upper Hutt River in Kaitoke Regional Park, New Zealand, was used as a location for the Fords of Isen scenes.