They had already won the battle over Sihon. Sihon was the king of the Amorites. He had ruled in Heshbon. Israel had also won the battle over Og at Edrei. Og was the king of Bashan. He had ruled in Ashtaroth.
“Then Israel sent messengers to Sihon. He was the king of the Amorites. He ruled in Heshbon. They said to him, ‘Let us pass through your country to our own land.’