Sihon, according to the Old Testament, was an Amorite king, who refused to let the Israelites pass through his country. The Bible describes that as the Israelites in their Exodus came to the country east of the Jordan, near Heshbon, King Siḥon of the Amorites refused to let them pass through his country:
"But Sihon would not allow Israel to pass through his territory. So Sihon gathered all his people together and went out against Israel in the wilderness, and he came to Jahaz and fought against Israel. Then Israel defeated him with the edge of the sword, and took possession of his land from the Arnon to the Jabbok, as far as the people of Ammon.." (Numbers 21:23-24)
Biblical historian Dr. Baden discusses the similarities between the encounter with Sihon and the earlier encounter with the king of Edom (Numbers 20:14-21) as well as a later parallel passage. (Deuteronomy 2:2-3:11) 
In a similar way the Israelites took the country of Og, and these two victories gave them possession of the complete country east of the Jordan, from the Arnon to the foot of the Hermon. These victories, among the earliest successful campaigns of the Israelites, quickly became legendary among them, and are referred to numerous times in the Bible as prototypical examples of God-given victory.