The Rohirrim come to Gondor’s aid when Minas Tirith is under attack by Sauron’s armies.
They are the horsemen of Rohan, led by King Théoden. After receiving Gondor’s call for help, Théoden gathers roughly 6,000 riders and secretly travels toward Minas Tirith. Guided through the hidden routes of the Drúadan Forest, they reach the Pelennor Fields at dawn, just after the Witch-king has broken the city’s main gate.
Their arrival changes the battle immediately. The Rohirrim sound their horns and charge the forces of Mordor, destroying siege engines, scattering orcs, and attacking the Haradrim. Théoden is killed during the fighting, but his nephew Éomer continues the assault. Later, Aragorn arrives with reinforcements from southern Gondor, helping turn the battle into a victory.
A common mix-up is choosing the Haradrim: they also fight at Minas Tirith, but they are allies of Sauron, not Gondor. The Rohirrim are also called the Riders of Rohan.