Sihon wouldn’t let Israel pass through his territory. Sihon gathered all his troops and came out into the desert to attack Israel. When Sihon’s troops came to Jahaz, they fought against Israel.
“The cry will be heard from Heshbon to Elealeh and Jahaz. It will be heard from Zoar to Horonaim and Eglath Shelishiyah. Even the streams of Nimrim will dry up.
I destroyed the Amorites in front of them, although the Amorites were as tall as cedars and as strong as oaks. I destroyed their fruit above the ground and their roots below it.
He gave these to the people when they were east of the Jordan River in the valley near Beth Peor, in the land of King Sihon of the Amorites, who ruled in Heshbon. Moses and Israel defeated him after they left Egypt.
But Sihon did not trust the Israelites enough to let them go through his territory. Sihon assembled all his troops. He camped at Jahaz and attacked Israel.