Numbers 21:26
For Heshbon was the city of Sihon the king of the Amorites, who had fought against the former king of Moab, and taken all his land out of his hand, even unto Arnon. For Heshbon was the city of Sihon the king of the Amorites, who had fought against the former king of Moab, and taken all his land out of his hand, even unto Arnon.
The verse belongs to the account of Israel's contest with Sihon and victory over Sihon and the Amorites as the people advanced toward Canaan, an advance that tested their faith. Unlike the earlier rebellious attempt to take the land without the appointed leader or the symbols of divine presence, Israel now moved forward at God's command with Moses and the divine presence leading them, and gained a complete victory. Heshbon was Sihon's Amorite city, previously taken from Moab.
By destroying the Amorites and conquering Bashan, Israel took possession of their land and gained rich territory. In doing so they proved themselves a powerful people camped in the plains of Moab on this side of the Jordan by Jericho, with only the Jordan separating them from the promised land.
When Balak king of the Moabites and all Moab learned of this, they were terrified and distressed, fearing Israel would consume everything around them, so Balak joined with the elders of Midian and sent for Balaam to curse Israel and drive them out. Balaam knew of Israel's recent triumph over Sihon and the divine power shown in those victories, and understood that no curse could harm Israel while they remained faithful to God.