The First and Second Battles of the Fords of Isen (which took place within a week of each other) were the opening skirmishes that fit into the overall campaign that Saruman planned to wage against Rohan in his quest to amass enough power to challenge Sauron. Tolkien wrote about these battles is greater detail, putting them into a broader context of the War of the Ring and providing a look into the military strategy used by both sides leading up to the Battle of the Hornburg at Helm's Deep. In the text, it is also explained how the forces of Rohan were ordered as well as where the named characters fit in the overall organization.
Having incapacitated King Theoden through the treachery of Grima Wormtongue, Saruman knew that his success in attacking Rohan depended on his ability to remove or neutralize two major obstacles: the King's son, Theodred, and his nephew, Eomer. Both of these hearty and devoted men were strong enough to lead the Rohirrim in Theoden's absence and they each recognized the need to temper Wormtongue's influence over the old man. Though Grima made subtle attempts to play them off one another, the two cousins could see through these games and were united in their loyalty to Theoden.
Saruman decided to launch an initial surprise attack on the Riders charged with protecting the western part of Rohan - the West-mark. The River Isen was essentially the Western border of the Mark. It flowed down from Isengard, out of the southernmost end of the Misty Mountains and down through the Gap of Rohan. It lay flat across the plains until it turned in a westward bend towards the coastal lands of southern Gondor. Just before this bend were the Fords of Isen where the river became broad and shallow. In the center of the stream was eyot, which had been built up and covered with stones. The water passed along either side of the eyot and was low enough for man and horse to cross it.
Once the river turned westwards, however, it flowed down long slopes and became deeper, turning into rapids. Crossing the river any farther south was virtually impossible and crossing any farther north was impractical. In the early days of Rohan, the Gap was more easily defended - from Isengard in the north and Helm's Deep in the south. And the narrow crossing in between would logistically limit an invading army's options for entering Rohan. But now, not only was the strength of Isengard no longer available to them, it was a threat against which they needed to watch diligently.
Though the idea of an attack from Isengard was anticipated by Rohan it was not expected to come as soon as it did. On February 25th, Theodred's scouts reported a force of Orcs massing southwards from Isengard, marching along the western side of the River Isen. Having assumed that this was Saruman's primary attack, Theodred led the majority of his cavalry across the Fords to intercept them, leaving a small contingency on the eastern bank. When the two opposing forces first met, the Rohirrim had initial success. However, it turned out to be a ruse. Several Orcs in entrenched positions attacked Theodred's flanks and forced the men to retreat. In the meantime, additional companies of Orcs supplemented by men of Dunland had crossed the river by way of a makeshift bridge just south of Isengard and proceeded down the eastern shore of the river. When Theodred reached the eyot midway across the river he saw that he was penned in from both sides and tried to make a stand with Grimbold at his side.
Now the Orcs had been instructed by Saruman to, above all else, find and kill Theodred. A group of ferocious Uruks assaulted the men on the eyot and Theodred was killed. Shortly after this, a group of riders led by Elfhelm arrived from Edoras and was able to drive back the Orcs, who felt they had already achieved their primary goal anyway. The Rohirrim were able to take back the Fords, but with heavy losses. The enemy, having been successful in killing Theodred retreated but here Saruman had made a huge tactical error. Had he ordered the force to continue across the Fords and attack Rohan's forces throughout the West-mark, then the men would have suffered many more losses and been scattered. Those that survived would have had little chance of being able to recover and regroup to continue the fight. Also, by delaying the main attack, he allowed Gandalf the opportunity to reach Edoras and convince Theoden to fight (a turn of events Saruman had not anticipated).
With Theodred dead, it fell to Erkenbrand to take command of the forces of the West-mark. But Erkenbrand was at Helm's Deep. When word had reached him of the news of battle, he knew that he would need to muster as many able-bodied men as he could from the surrounding lands before setting out for the Fords. He sent a message back that Grimbold was to be in charge of Theodred's forces until he could join them. All was in disarray and Erkenbrand, sensing the strategic importance of the Hornburg, was wary of leaving it unprotected. He also sent word down to Edoras, calling for Eomer to set out at once with the army of the East-mark.
On February 27th, Grima convinced Theoden to deny Erkenbrand's request on the grounds that it would leave Edoras vulnerable to attack. Eomer was forbidden to leave the capital. But there was a report that a band of Orcs had descended the western Emyn Muil and was heading across the plains towards Isengard. Disobeying orders, Eomer set out with a company of riders to dispatch them, which they did on the 29th (remember in Middle-Earth's calendar February has 30 days). On the 30th of February, Eomer encountered Aragorn, Legolas and Gimli on his way back to Edoras.
By March 2nd, Elfhelm and Grimbold were able to regroup the scattered Rohirrim. Grimbold was in command of the Theodred's forces and Elmhelm lead the muster of Edoras. There were in a disagreement over how to best defend against another attack. Elmhelm believed that the Fords were nothing more than a trap at that point since Saruman's troops had shown that they could come from both sides at once. Grimbold, however, was concerned that if Saruman became aware of the state of Rohan's forces he might send another stronger army of Orcs to cross at the Fords and attack them from the rear. In the end they compromised. Grimbold and his men guarded the west bank of the Fords and Elfhelm positioned his Riders on the eastern side but farther north to be able to spot an attack from that side as soon as possible.
Later that day, as Gandalf freed King Theoden from Grima's influence, the Second Battle of the Fords of Isen began. An army of Orcs came down the road from Isengard and engaged Grimbold's troops. By sunset he was forced to retreat to the eastern side of the Fords. But the Orcs did not yet attempt to cross. Errand riders were dispatched to the Hornburg to alert Erkenbrand of the gravity of the situation and to urge him to make haste to the Fords with whatever force he had already mustered. Elfhelm was not yet aware of the skirmish at the Fords and just before midnight, the massive force which emptied itself out of Isengard under the watch of the Ents, made its presence known with hundreds of lit torches moving swiftly down the eastern side of the river.
The Orcs on the western bank, seeing the torches of the other army in the distance, poured over the Fords and attacked the Rohirrim in a pincer movement. Grimbold attempted a maneuver in which half his forces retreated and then swung around to the Orcs' flanks from the north and south - giving the impression that reinforcements had arrived. While it seemed to work at first, it ultimately had the effect of scattering the men. Wolf-riders charged north to attack the rear of Elfhelm's army, which was now preparing for the assault from the other direction. The results were disastrous. Like Grimbold's army, Elfhelm's forces suffered heavy losses with the remaining men being scattered. Erkenbrand's men had left Helm's Deep but not yet arrived.
Theoden and Gandalf led one thousand Riders north to answer the call that had been thwarted by Grima days before. Heading in the direction of the Fords on March 3rd, they encountered the scout Ceorl who reported to the King of the second Orc attack at the Fords. As described in Chapter VII of Book Three ("Helm's Deep"), Theoden was prepared to continue on to reinforce his men at the river but Gandalf implored him to instead turn the Riders towards Helm's Deep and prepare for an assault on the Hornburg. Gandalf was sure this was Saruman's ultimate objective but needed to gather more information and find out what happened to the rest of the Rohirrim.
Gandalf first rode swiftly north upon Shadowfax until he reached Isengard. He met briefly with Treebeard and learned that all of Saruman's army had gone. Assured that the Ents had the situation there under control, he rode again southwards and gathered the remaining forces of Elfhelm and Grimbold to join Erkenbrand. Once united, they began a march back towards Helm's Deep. They arrived at daybreak on March 4th, just in time to turn the tide of battle and drive the Orcs into the forest of Huorns that now blocked all access out of the Deeping-coomb.
In an appendix to the essay, Christopher Tolkien explains how the armies of Rohan were set-up under the control of the King. The highest military rank was Marshal and throughout Rohan's history there were always three of them. The First Marshal of the Mark was charged with protecting Edoras and the adjacent lands, including Dunharrow. The Second Marshal's territory was the West-mark and was based at Helm's Deep. The Third Marshal commanded the East-mark was operated out of the original House of Eorl the Young at Aldburg.
The Second and Third Marshals at the time of the War of the Ring were Theodred and Eomer, respectively. Since Theoden assumed the throne at the young age of 32, no one was appointed First Marshal. The role of protector of the capitol was assumed by the King himself. But throughout most of his reign there was peace and little need for him to ride to war. In the years 3012-3019 of the Third Age, much of the duties of First Marshal fell to Elfhelm though he never officially had the title. As Theoden grew old and seemingly decrepit there was a breakdown in communications between Edoras and the two Marshals and barely any central command (a situation Grima did everything he could to maintain).
Theoden again took command of the Muster of Edoras and after the Battle of Helm's Deep he answered the call of Gondor. Being as Theodred had been killed, he took Eomer as his right hand and successor. Though the title of First Marshal was never used, the King's nephew became - for all intents and purposes - the First Marshal of the Mark. Though Erkenbrand technically outranked Elfhelm and Grimbold, the King sent him back to Edoras to protect the civilian population. He would serve as King if both Theoden and Eomer did not survive. At the Battle of the Pelennor Fields, Elfhelm and Grimbold served unofficially as the Second and Third Marshals with each commanding his own force. Grimbold died in that battle.
After the death of Theoden, Eomer assumed the throne and reordered the realm so that it had only two Marshals. Erkenbrand became Marshal of the West-mark and Elfhelm was appointed Marshal of the East-mark.
Unlike the past Marshals, which were ranked First, Second and Third, neither of these two had precedence over the other.