Nahal Arnon

The border between the Emori kingdom of Sihon and Moav. When Bnei Yisrael approached Eretz Yisrael, they were prohibited from attacking Moav, but conquered Sihon's kingdom from Nahal Arnon northward. Hundreds of years later, Yiftah explained this point to the King of Ammon who refused to accept it and chose war instead. Nahal Arnon is the southern border of Reuven's tribal territory, but later it apparently became part of Moav's territory.