In the thirteenth century of the Third Age (T.A.), the Kings of Gondor made close alliances with the Northmen of Rhovanion, a people said in The Lord of the Rings to be akin to the Three Houses of Men (later the Dúnedain) from the First Age.
In the twenty-first century, 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 2509, Cirion the Steward of Gondor sent summons to the Éothéod for aid in throwing off a combined invasion of Men from the north east of Middle-earth, and Orcs from Mordor.
Eorl the Young, king of the Éothéod, answered the summons, and arrived unexpected at a decisive battle at the Field of Celebrant, routing the orc army, and then destroying it as it fled.
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 plague of 1636, and the survivors to a large extent slain in the invasion mentioned above.
The first line of kings lasted for 249 years, until the ninth king Helm Hammerhand died. His sons had been killed earlier, and his nephew Fréaláf Hildeson began the second line of kings, which lasted until the end of the Third Age.
In 2758, Rohan was invaded by Dunlendings under Wulf, son of Freca, of mixed Dunland and Rohan blood. The King, Helm Hammerhand, took refuge in the Hornburg until aid from Gondor and Dunharrow (a refuge of the Rohirrim) arrived a year later and defeated the invaders.
It was soon after this that Saruman arrived and took over Isengard, and was welcomed as a strong ally, since it would take Rohan close to 200 years to recover its strength after the invasion.
In 3014, Saruman began using his influence to weaken the King, Théoden, as part of a campaign to invade or take over the kingdom. In 3019, he launched a great invasion of Rohan, with victory in the two first battles (at the Fords of Isen; Théoden's son, Théodred was killed during these attacks) and defeat at the Battle of the Hornburg, where the Huorns came to the aid of the Rohirrim.
On the heels of this victory, Théoden rode with an army to Minas Tirith and helped break its siege in the Battle of the Pelennor Fields, where he was slain. Éomer, the nephew of King Théoden, then took up the reign, beginning the third line. Éomer rode with the armies of Gondor to the Black Gate of Mordor and took part in the Battle of the Morannon against the forces of Sauron, who were defeated when the Ruling Ring was destroyed.
The rule of the Stewards of Gondor was then over. King Éomer and the new king of Gondor, Elessar, renewed their oath of alliance, and reaffirmed Cirion's grant of Calenardhon to the Rohirrim.
In the Fourth Age, Rohan remained in peaceful coexistence with the Reunited Kingdom. It became the site of routes where Elves migrated from their eastern kingdoms to Lindon to leave Middle-earth. A few remained behind to help in the reconstruction of Rohan. A Dwarven community developed in the caves of Helm's Deep, which became prosperous from its mining of precious materials.
