Judges 11:20

But Sihon trusted not Israel to pass through, his coast
For fear they should seize upon his dominions, and retain them; and the more fearful he might be, as he knew that his people were one of the seven nations of the Canaanites, whose land they were going to possess, and whom they were to destroy:

but Sihon gathered all his people together;
in some certain place, and armed them, and went out in an hostile manner against Israel in the wilderness, to attack them; whereby it appears that he was the aggressor, and therefore Israel was not to be blamed, as not for fending themselves, so neither for seizing and possessing his country when they had conquered him:

and pitched in Jahaz, and fought against Israel;
there was a battle between them at the place mentioned, and the victory was on Israel's side, see ( Numbers 21:23-35 ) .