Under the Arsacid Empire, the Great Wall of Gorgan, a series of forts and outposts with the plains of Hyrcania, was constructed to aid in the defence of Hyrcania against raids undertaken by the neighbouring Dahae tribes. At the beginning of the Roman–Parthian War of 58–63, a rebellion erupted in Hyrcania and rebels sent envoys to Emperor Nero requesting aid. The rebellion raged until 60 AD when Vologases I hastily concluded a peace treaty with the rebels to allow him to deal with the threat posed by the Romans. However, the peace treaty did not last and the Hyrcanians launched another revolt that continued until at least 75 AD. In 75 AD, the Hyrcanian rebels allied with nomadic Alan tribes and granted them safe passage through Hyrcania into Media, allowing the Alans to pillage Media and Armenia. In the time of the reign of the Emperor Antoninus Pius (138–161), Hyrcania had made itself independent and was not considered part of the Arsacid Empire. Hyrcania is mentioned as Li-chien (or Li-kan, 黎幹) in the 2nd century AD Book of Han.