Gondor was part of the Last Alliance of Elves and Man which overthrew Sauron for the first time. After the war Gondor's power and wealth grew steadily (only interrupted by an Easterling invasion in 492 Third Age). In the reign of the powerful king Hyarmendacil I (12th century T.A.) Gondor reached the height of its power.
But after his reign decadence spread under the kings of Gondor and a long period of decline began (although Gondor experienced several revivals). In the 15th century a great civil war named the Kin-strife tore the nation apart.
In 1944 T.A. Gondor faced a constitutional crisis when King Ondoher was slain in battle without heirs. Arvedui of Arthedain and a victorious general named Eärnil claimed the throne. Steward Pelendur intervened in favor of the latter, who reigned as Eärnil II.
The realm was governed by a long line of stewards after the disappearance of Eärnur, son of Eärnil, since there was no proof that the last king was dead, and no claimant had enough support to be accepted as his successor.
In 2510 T.A. when Steward Cirion ruled over Gondor, the nation faced one of its greatest perils, an Easterling tribe named the Balchoth invaded Gondor with mass force. Gondor's army marched to fight the Balchoth but were cut off from Minas Tirith and pushed back in the direction of the Limlight.
Messengers were sent to get help from the Éothéod, a tribe which lived in the northern vales of the Anduin, but nobody expected the messengers to reach their destination. When certain peril came upon Gondor, however, the Rohirrim turned the tide of the Battle of the Field of Celebrant. After the victory the Éothéod were awarded the fields of Calenardhon north of the Ered Nimrais from the gap of Rohan at the southern end of the Hithaeglir, Fangorn forest, rivers Limlight to river Anduin, western Emyn Muil and river Mehring, where they established the kingdom of Rohan with Eorl the Young as their first king. A permanent alliance between Gondor and Rohan was established by the oath Eorl swore to Cirion.
In 3019 Gondor faced an all out attack on its capital, Minas Tirith, in the Battle of the Pelennor Fields. Although nearly defeated the Rohirrim once again turned the tide of battle.