This map shows the armies of Saruman in black, and the main force of the Rohirrim (commanded by Grimbold of Westfold) in white. Green represents the Rohirrim of the Muster of Edoras, commanded by Elfhelm, who ultimately played little direct part in the battle.

(1)	Toward noon, a forward element of Saruman's forces comes down the road from Isengard on the western side of Isen and assaults the forts guarding the Fords of Isen, which the Rohirrim defend.
(2)	Anticipating that a secondary force will approach down the eastern side of Isen, Elfhelm deploys his Riders to screen the defenders against any attack from that direction.
(3)	Saruman's forces push the defenders back from the forts on the western bank, but Grimbold is able to maintain the defence of the eastern bank.
(4)	Under the darkness of night, a much greater army comes down the road out of the north and crosses the Fords. Grimbold is forced to abandon the eastern bank and fall back to his camp.
(5)	Grimbold holds the enemy at bay using a shieldwall. At his orders, his surviving cavalry under Dúnhere break out of the camp and briefly harass the enemy before riding off into the plain.
(6)	As Elfhelm had predicted, a second force comes down the river on its eastern bank, but the Rohirrim are flanked by wolfriders, and forced to make their retreat eastward.
(7)	Having defeated and driven off the Rohirrim, Saruman's combined forces continue down the road into Westfold, and towards the Battle of the Hornburg, which will be fought on the following night.

The second of the two Battles of the Fords of Isen, fought by the forces of Isengard to weaken the western defences of Rohan and prepare the way for Saruman's full invasion of the Mark. The First Battle had been fought on 25 February III 3019, and while it had ended indecisively from a military standpoint, it had seen the death of Théodred, the heir of King Théoden and one of Rohan's most capable commanders.

Seven days after the First Battle, on 2 March, Saruman's invaders once again approached the Fords of Isen. Where the First Battle had been little more than a raid, the White Wizard now committed two great armies, ready to sweep aside the defenders and drive on into Rohan. Those defenders comprised Riders of Westfold led by Grimbold, as well as a contingent from Edoras who had formed the guard of fallen Théodred, and were now commanded by Elfhelm.

The Battle

The site of the battle presented a particular challenge for the defenders, because Isengard stood near the sources of Isen, and so Saruman had the luxury of sending his forces down either side of the river. Isen flowed southward through the Fords, so Elfhelm set up a defensive screen on the eastern side, while Grimbold's warriors were deployed on the river's western bank.

The Sun was high in the sky when the battle began, with a force from Isengard marching down the western bank of Isen. Though they represented only a part of Saruman's full forces, they were sufficient to push Grimbold back from the forts on the western side of the river. Grimbold's fighters were forced back onto the eastern bank, and the Fords of Isen and their eyot were taken by Saruman's Uruks. As the Sun fell toward the horizon, Grimbold and his Men still held the eastern bank of the river, and a pause came to the fighting. In this respite, Elfhelm moved his own soldiers southward, ready to reinforce Grimbold, while still maintaining his guard against a possible secondary assault down Isen's eastern bank.

As the Rohirrim fought desperately to hold the Fords, Saruman released his entire invasion army, a force far greater than any the Riders had yet faced.¹ Many of these came down the western side of the river to redouble the assault on the Crossings, while others travelled stealthily down the eastern side. Elfhelm's soldiers had been placed to intercept such an eastern force, but they were not ready for the scale of the attack. Wolfriders ran in behind Elfhelm's defenders, splitting him from Grimbold to the south. Meanwhile the invaders from the west rushed the Fords, pushing back Grimbold from the eastern bank so that he was forced to retreat to his camp, setting a shield-wall as a last line of defence.

It was now deep night, and both banks of the Fords had been taken by the enemy. Grimbold's remaining horsemen, led by Dúnhere, broke out of the ring of shields and launched a last attack against the enemy, before riding away - as ordered - into the night. Meanwhile, with the Fords secured, Saruman's commander marched his armies on southward into Rohan, while the last defenders were forced into retreat in scattered disarray across the land. So the Second Battle of the Fords of Isen ended in defeat for the Rohirrim.

After the Battle

The battle was lost, but Rohan still had a last line of defence at the fortress of the Hornburg in Helm's Deep, and many of the survivors of the battle made their way to Erkenbrand, who commanded the fortress. One of these survivors, a Rider named Ceorl was able to find the King as he rode out from Edoras, and redirect him to the Hornburg. Meanwhile Gandalf found both Grimbold and Elfhelm, and had them rally their forces. All these elements combined at the Battle of the Hornburg, the night after the Second Battle of the Fords of Isen, in which Saruman's invaders were utterly defeated.