Rohan, originally Rochand, was a kingdom of herdsmen and some farmers on the northern borders of Gondor in Middle-earth. Well-known for their horses and cavalry, they are Gondor's most important ally.
In the 2000s, a remnant tribe of such Northmen calling itself the Éothéod moved from the valleys of Anduin to the north west of Mirkwood, clearing out what remained of the recently defeated witch Kingdom of Angmar, east of the Misty Mountains. While there, some dispute arose between them and the dwarves over the treasure-hoard of Scatha the dragon.
Later, in TA 2509, Cirion the Steward of Gondor sent summons to the Éothéod for aid in throwing off a combined invasion of Men from the Northeast of Middle-earth, and Orcs from Mordor. Eorl the Young, king of the Éothéod, answered the summons, and arrived unexpected at a decisive Battle of the Field of Celebrant. As a reward, Eorl was given the plains of Calenardhon, and he moved his kingdom there. This land had earlier been part of Gondor proper, but had been devastated by the Great Plague of TA 1636, and the survivors to a large extent slain in the invasion.