The Battle of Reach was a planetary battle that took place during 2555. There was no main reason for the battle, as each side had a very different reason. It mainly took place in the city of Terrador, which had survived the Fall of Reach. In the end, the Jiralhanae were nearly annihilated, the Sangheili retreated and Tir-D-yar's forces conquered Reach for their own. This battle eventually lead to the creation of Luafor, the city constructed by Lady Sintharia'sNartol.
Fall of the Jiralhanae
The Battle of Reach was the main storyline for RP:Reach, however it's name is never stated.