Calenardhon was a wide green landscape in the north of Gondor, but it became depopulated during the Third Age. During the rule of the StewardCirion Gondor faced a series of attacks by the Balchoth, and Calenardhon was overrun by these evil people. Cirion sent messages for help to Gondor's old allies, including the Éothéod, to help defend Gondor.
Eorl the Young answered the call, and helped Gondor defeat the Balchoth at the Field of Celebrant. In gratitude, Cirion granted all Calenardhon, except for Anórien, to the Éothéod.
After the Éothéod settled in Calenardhon, the land became known as Rohan or the Riddermark.
In 2510 the Balchoth crossed Anduin on numerous lands, passing into the Wold in the north of the Gondorian province of Calenardhon.
They met little resistance as Calenardhon had been mostly abandoned long before, and by the time the North Army of Gondor appeared from the fortresses of Isengard, Aglarond and other fortresses in the White Mountains the bulk of the Balchoth army had crossed.