The Dunlendings were so-called Middle Men like the Rohirrim, but since they spoke a different language (related to the the Second House of Men, the Haladin, rather than the Bëorian/Marachian tongue which stood at the base of Adûnaic), they were not recognised as Middle Men by the Númenoreans of the Second Age. Originally the inhabitants of the forested region of Enedwaith, they were grouped with the Minhiriathrim, another people which were related to the Haladin rather than the Bëorians or Marachians. This lack of understanding between them and the Númenoreans later led to outright hostility as the Númenoreans began cutting down the large forests, and began driving the Dunlendings away from their ancestral lands. The Dunlendings fled to the Misty Mountains, the cape of Eryn Vorn, and the White Mountains.
The Dunlendings of northern Eriador mingled with other Middle Men, becoming the Men of Bree and other Men of Arnor, forgetting their ancestry and tongue, becoming subjects of the Dúnedain of Arnor. But the Dunlendings of Eryn Vorn and the White Mountains remained hostile. The Dead Men of Dunharrow were descended of these people. In the White Mountains the Dunlendings drove out the Druedain or Woses, until they in turn were driven out by the Dúnedain when the realm of Gondor was established, extending on to Enedwaith.
Guarding the Gap of Rohan was the fortress of Isengard, where a hereditary guard watched for Gondor. However, by the time of the Steward of Gondor Beren, these guards had mixed with Dunlendings, and it had become hostile to Gondor. To remedy this situation, Beren gave Saruman the keys to Orthanc, to guard Isengard for Gondor.
When Gondor decided to give the unpopulated province of Calenardhon to the numerous people of Éothéod in 2510 T.A., the Dunlendings felt threathened by these Strawheads (referring to the blonde hair). This because they had slowly reclaimed their land during the dwindling of the Dúnedain, having reclaimed all the land between the rivers Greyflood and Isen, and beginning to creep back in Calenardhon itself. Still, open war was not waged until the reign of Helm Hammerhand (2741 T.A. - 2759 T.A.). The Dunlending Freca tried to get the throne of Rohan for himself by letting his son Wulf marry the daughter of Helm. Freca was killed, and Wulf led the Dunlendings into open war with Rohan. They unsuccesfully besieged the Hornburg during the long winter of 2758-2759. Wulf did take Edoras and killed Haleth, the son of Helm, in front of the golden hall of Meduseld. But in the refuge of Dunharrow Helm's nephew Fréalaf held out against the Dunlendings. He recaptured Edoras in the end of the long winter and killed Wulf personally. The Dunlendings were driven out of Rohan, and Fréalaf succeeded the deceased Helm Hammerhand.
Saruman used this old history to tempt the Dunlendings in supporting him.
After the battle at Helm's Deep the Dunlendings were allowed to live by the Rohirrim, on the condition that all hostilites ceased, and they retreated behind the Isen river again.