From Tolkien Gateway

The West-mark was the military division of Rohan that lay to the west of the Entwash and Snowbourn rivers (excepting the lands around the capital at Edoras), commanded by a Marshal based at Helm's Deep.[1] Bordering on Rohan's enemies in Dunland, the West-mark was historically the scene of much strife. The worst troubles in this region were seen during the Long Winter, when the Dunlendings forced the Rohirrim to abandon Edoras and flee to the West-mark, where they sheltered in Helm's Deep.[2]

Isengard also lay on its borders, so that the West-mark was of great importance during the War of the Ring. The King's son Théodred was given the rank of Second Marshal of the Mark, and given the command of this region in the wars with Saruman. He went to battle with the enemy at the Fords of Isen, where he lost his life. The command of the West-mark was then taken by the lord Erkenbrand, who fought and lost a second battle at the Fords, but was able to rally enough of his forces to help in the desperate defence of Helm's Deep.[3]

See also