Whether you are a fan of the books or the movies or both, we all know this scene. But we only get a little history behind the relationship between Gondor and Rohan. This section discusses the beginning of the alliance.
The Eothed were Northman who were great breeders of horses. They are the same race of men who became allies of the Eldar against Morgoth. Then came the Great Plague, which reduced the strength of the Eothed and Gondor. Then the Wainriders invaded, they were a tribe of Easterlings. Most of the Northmen were reduced to servitude, took refuge in Gondor, or fled to Dale.
In 2489, Cirion became the Steward of Gondor.
“For he was a man of little pride and of great courage and generosity of heart, the noblest of the Stewards of Gondor…”The Unfinished Tales
He was soon made aware of the plans to invade Gondor. The only way to survive was to get the help of the Eothed and a message was sent asking for help. Just when all hope seemed lost, Eorl, the son of Leod of the Eothed led a group of 7,000 to help Gondor. After the battle, Cirion led Eorl to Halifirien, also known as the Hill of Awe and where the highest beacon was built. They then made the Oath of Eorl and the Oath of Cirion at the Tomb of Elendil.
“There, if he will, he shall be kind, and his heirs after him, and his people shall dwell in Freedom while the authority of the Stewards endures until the Great King returns.”The Unfinished Tales
Cirion gives Eorl and his people the land of Calenardhon, from Anduin to Isen and there they grew in numbers and power. Eorl took the title, King of the Mark. Mark meaning borderlands. In Sindarin, Rohan means the Mark, and Rohhirim means the people.